diff --git a/include/random.h b/include/random.h
index 6bf61de6c..60c718348 100644
--- a/include/random.h
+++ b/include/random.h
@@ -20,4 +20,67 @@ u16 Random2(void);
 void SeedRng(u16 seed);
 void SeedRng2(u16 seed);
 
+/* Structured random number generator.
+ * Instead of the caller converting bits from Random() to a meaningful
+ * value, the caller provides metadata that is used to return the
+ * meaningful value directly. This allows code to interpret the random
+ * call, for example, battle tests know what the domain of a random call
+ * is, and can exhaustively test it.
+ *
+ * RandomTag identifies the purpose of the value.
+ *
+ * RandomUniform(tag, lo, hi) returns a number from lo to hi inclusive.
+ *
+ * RandomPercentage(tag, t) returns FALSE with probability (1-t)/100,
+ * and TRUE with probability t/100.
+ *
+ * RandomWeighted(tag, w0, w1, ... wN) returns a number from 0 to N
+ * inclusive. The return value is proportional to the weights, e.g.
+ * RandomWeighted(..., 1, 1) returns 50% 0s and 50% 1s.
+ * RandomWeighted(..., 2, 1) returns 2/3 0s and 1/3 1s. */
+
+enum RandomTag
+{
+    RNG_NONE,
+    RNG_ACCURACY,
+    RNG_CONFUSION,
+    RNG_CRITICAL_HIT,
+    RNG_CUTE_CHARM,
+    RNG_DAMAGE_MODIFIER,
+    RNG_FLAME_BODY,
+    RNG_FORCE_RANDOM_SWITCH,
+    RNG_FROZEN,
+    RNG_HOLD_EFFECT_FLINCH,
+    RNG_INFATUATION,
+    RNG_PARALYSIS,
+    RNG_POISON_POINT,
+    RNG_RAMPAGE_TURNS,
+    RNG_SECONDARY_EFFECT,
+    RNG_SLEEP_TURNS,
+    RNG_SPEED_TIE,
+    RNG_STATIC,
+    RNG_STENCH,
+};
+
+#define RandomWeighted(tag, ...) \
+    ({ \
+        const u8 weights[] = { __VA_ARGS__ }; \
+        u32 sum, i; \
+        for (i = 0, sum = 0; i < ARRAY_COUNT(weights); i++) \
+            sum += weights[i]; \
+        RandomWeightedArray(tag, sum, ARRAY_COUNT(weights), weights); \
+    })
+
+#define RandomPercentage(tag, t) \
+    ({ \
+        const u8 weights[] = { 100 - t, t }; \
+        RandomWeightedArray(tag, 100, ARRAY_COUNT(weights), weights); \
+    })
+
+u32 RandomUniform(enum RandomTag, u32 lo, u32 hi);
+u32 RandomWeightedArray(enum RandomTag, u32 sum, u32 n, const u8 *weights);
+
+u32 RandomUniformDefault(enum RandomTag, u32 lo, u32 hi);
+u32 RandomWeightedArrayDefault(enum RandomTag, u32 sum, u32 n, const u8 *weights);
+
 #endif // GUARD_RANDOM_H
diff --git a/src/battle_script_commands.c b/src/battle_script_commands.c
index bd1b248b2..283adb54a 100644
--- a/src/battle_script_commands.c
+++ b/src/battle_script_commands.c
@@ -1918,15 +1918,25 @@ static void Cmd_accuracycheck(void)
     }
     else
     {
+        u32 accuracy;
+
         GET_MOVE_TYPE(move, type);
         if (JumpIfMoveAffectedByProtect(move))
             return;
         if (AccuracyCalcHelper(move))
             return;
 
-        // final calculation
-        if ((Random() % 100 + 1) > GetTotalAccuracy(gBattlerAttacker, gBattlerTarget, move, GetBattlerAbility(gBattlerAttacker), GetBattlerAbility(gBattlerTarget),
-                                                    GetBattlerHoldEffect(gBattlerAttacker, TRUE), GetBattlerHoldEffect(gBattlerTarget, TRUE)))
+        accuracy = GetTotalAccuracy(
+            gBattlerAttacker,
+            gBattlerTarget,
+            move,
+            GetBattlerAbility(gBattlerAttacker),
+            GetBattlerAbility(gBattlerTarget),
+            GetBattlerHoldEffect(gBattlerAttacker, TRUE),
+            GetBattlerHoldEffect(gBattlerTarget, TRUE)
+        );
+
+        if (!RandomPercentage(RNG_ACCURACY, accuracy))
         {
             gMoveResultFlags |= MOVE_RESULT_MISSED;
             if (GetBattlerHoldEffect(gBattlerAttacker, TRUE) == HOLD_EFFECT_BLUNDER_POLICY)
@@ -2105,10 +2115,8 @@ static void Cmd_critcalc(void)
         gIsCriticalHit = FALSE;
     else if (critChance == -2)
         gIsCriticalHit = TRUE;
-    else if (Random() % sCriticalHitChance[critChance] == 0)
-        gIsCriticalHit = TRUE;
     else
-        gIsCriticalHit = FALSE;
+        gIsCriticalHit = RandomWeighted(RNG_CRITICAL_HIT, sCriticalHitChance[critChance] - 1, 1);
 
     // Counter for EVO_CRITICAL_HITS.
     partySlot = gBattlerPartyIndexes[gBattlerAttacker];
@@ -3154,9 +3162,9 @@ void SetMoveEffect(bool32 primary, u32 certain)
 
             if (sStatusFlagsForMoveEffects[gBattleScripting.moveEffect] == STATUS1_SLEEP)
             #if B_SLEEP_TURNS >= GEN_5
-                gBattleMons[gEffectBattler].status1 |= ((Random() % 3) + 2);
+                gBattleMons[gEffectBattler].status1 |= STATUS1_SLEEP_TURN(1 + RandomUniform(RNG_SLEEP_TURNS, 1, 3));
             #else
-                gBattleMons[gEffectBattler].status1 |= ((Random() % 4) + 3);
+                gBattleMons[gEffectBattler].status1 |= STATUS1_SLEEP_TURN(1 + RandomUniform(RNG_SLEEP_TURNS, 2, 5));
             #endif
             else
                 gBattleMons[gEffectBattler].status1 |= sStatusFlagsForMoveEffects[gBattleScripting.moveEffect];
@@ -3559,7 +3567,7 @@ void SetMoveEffect(bool32 primary, u32 certain)
                 {
                     gBattleMons[gEffectBattler].status2 |= STATUS2_MULTIPLETURNS;
                     gLockedMoves[gEffectBattler] = gCurrentMove;
-                    gBattleMons[gEffectBattler].status2 |= STATUS2_LOCK_CONFUSE_TURN((Random() & 1) + 2); // thrash for 2-3 turns
+                    gBattleMons[gEffectBattler].status2 |= STATUS2_LOCK_CONFUSE_TURN(RandomUniform(RNG_RAMPAGE_TURNS, 2, 3));
                 }
                 break;
             case MOVE_EFFECT_SP_ATK_TWO_DOWN: // Overheat
@@ -3780,20 +3788,23 @@ static void Cmd_seteffectwithchance(void)
     else
         percentChance = gBattleMoves[gCurrentMove].secondaryEffectChance;
 
-    if (gBattleScripting.moveEffect & MOVE_EFFECT_CERTAIN
-        && !(gMoveResultFlags & MOVE_RESULT_NO_EFFECT))
+    if (!(gMoveResultFlags & MOVE_RESULT_NO_EFFECT)
+     && gBattleScripting.moveEffect)
     {
-        gBattleScripting.moveEffect &= ~MOVE_EFFECT_CERTAIN;
-        SetMoveEffect(FALSE, MOVE_EFFECT_CERTAIN);
-    }
-    else if (Random() % 100 < percentChance
-             && gBattleScripting.moveEffect
-             && !(gMoveResultFlags & MOVE_RESULT_NO_EFFECT))
-    {
-        if (percentChance >= 100)
+        if (gBattleScripting.moveEffect & MOVE_EFFECT_CERTAIN
+         || percentChance >= 100)
+        {
+            gBattleScripting.moveEffect &= ~MOVE_EFFECT_CERTAIN;
             SetMoveEffect(FALSE, MOVE_EFFECT_CERTAIN);
-        else
+        }
+        else if (RandomPercentage(RNG_SECONDARY_EFFECT, percentChance))
+        {
             SetMoveEffect(FALSE, 0);
+        }
+        else
+        {
+            gBattlescriptCurrInstr = cmd->nextInstr;
+        }
     }
     else
     {
@@ -12344,7 +12355,7 @@ static void Cmd_forcerandomswitch(void)
             *(gBattleStruct->battlerPartyIndexes + gBattlerTarget) = gBattlerPartyIndexes[gBattlerTarget];
             gBattlescriptCurrInstr = BattleScript_RoarSuccessSwitch;
             gBattleStruct->forcedSwitch |= gBitTable[gBattlerTarget];
-            *(gBattleStruct->monToSwitchIntoId + gBattlerTarget) = validMons[Random() % validMonsCount];
+            *(gBattleStruct->monToSwitchIntoId + gBattlerTarget) = validMons[RandomUniform(RNG_FORCE_RANDOM_SWITCH, 0, validMonsCount - 1)];
 
             if (!IsMultiBattle())
                 SwitchPartyOrder(gBattlerTarget);
diff --git a/src/battle_util.c b/src/battle_util.c
index 3e8f94bf6..7bf090a73 100644
--- a/src/battle_util.c
+++ b/src/battle_util.c
@@ -3479,7 +3479,7 @@ u8 AtkCanceller_UnableToUseMove(void)
         case CANCELLER_FROZEN: // check being frozen
             if (gBattleMons[gBattlerAttacker].status1 & STATUS1_FREEZE && !(gBattleMoves[gCurrentMove].flags & FLAG_THAW_USER))
             {
-                if (Random() % 5)
+                if (!RandomPercentage(RNG_FROZEN, 20))
                 {
                     gBattlescriptCurrInstr = BattleScript_MoveUsedIsFrozen;
                     gHitMarker |= HITMARKER_NO_ATTACKSTRING;
@@ -3597,9 +3597,9 @@ u8 AtkCanceller_UnableToUseMove(void)
                 {
                      // confusion dmg
                 #if B_CONFUSION_SELF_DMG_CHANCE >= GEN_7
-                    if (Random() % 3 == 0)
+                    if (RandomWeighted(RNG_CONFUSION, 2, 1))
                 #else
-                    if (Random() % 2 == 0)
+                    if (RandomWeighted(RNG_CONFUSION, 1, 1))
                 #endif
                     {
                         gBattleCommunication[MULTISTRING_CHOOSER] = TRUE;
@@ -3625,7 +3625,7 @@ u8 AtkCanceller_UnableToUseMove(void)
             gBattleStruct->atkCancellerTracker++;
             break;
         case CANCELLER_PARALYSED: // paralysis
-            if ((gBattleMons[gBattlerAttacker].status1 & STATUS1_PARALYSIS) && (Random() % 4) == 0)
+            if ((gBattleMons[gBattlerAttacker].status1 & STATUS1_PARALYSIS) && !RandomPercentage(RNG_PARALYSIS, 75))
             {
                 gProtectStructs[gBattlerAttacker].prlzImmobility = TRUE;
                 // This is removed in FRLG and Emerald for some reason
@@ -3640,7 +3640,7 @@ u8 AtkCanceller_UnableToUseMove(void)
             if (gBattleMons[gBattlerAttacker].status2 & STATUS2_INFATUATION)
             {
                 gBattleScripting.battler = CountTrailingZeroBits((gBattleMons[gBattlerAttacker].status2 & STATUS2_INFATUATION) >> 0x10);
-                if (Random() & 1)
+                if (!RandomPercentage(RNG_INFATUATION, 50))
                 {
                     BattleScriptPushCursor();
                 }
@@ -5645,7 +5645,7 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u16 ability, u8 special, u16 move
              && TARGET_TURN_DAMAGED
              && CanBePoisoned(gBattlerTarget, gBattlerAttacker)
              && IsMoveMakingContact(move, gBattlerAttacker)
-             && (Random() % 3) == 0)
+             && RandomWeighted(RNG_POISON_POINT, 2, 1))
             {
                 gBattleScripting.moveEffect = MOVE_EFFECT_AFFECTS_USER | MOVE_EFFECT_POISON;
                 PREPARE_ABILITY_BUFFER(gBattleTextBuff1, gLastUsedAbility);
@@ -5663,7 +5663,7 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u16 ability, u8 special, u16 move
              && TARGET_TURN_DAMAGED
              && CanBeParalyzed(gBattlerAttacker)
              && IsMoveMakingContact(move, gBattlerAttacker)
-             && (Random() % 3) == 0)
+             && RandomWeighted(RNG_STATIC, 2, 1))
             {
                 gBattleScripting.moveEffect = MOVE_EFFECT_AFFECTS_USER | MOVE_EFFECT_PARALYSIS;
                 BattleScriptPushCursor();
@@ -5679,7 +5679,7 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u16 ability, u8 special, u16 move
              && (IsMoveMakingContact(move, gBattlerAttacker))
              && TARGET_TURN_DAMAGED
              && CanBeBurned(gBattlerAttacker)
-             && (Random() % 3) == 0)
+             && RandomWeighted(RNG_FLAME_BODY, 2, 1))
             {
                 gBattleScripting.moveEffect = MOVE_EFFECT_AFFECTS_USER | MOVE_EFFECT_BURN;
                 BattleScriptPushCursor();
@@ -5695,7 +5695,7 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u16 ability, u8 special, u16 move
              && (IsMoveMakingContact(move, gBattlerAttacker))
              && TARGET_TURN_DAMAGED
              && gBattleMons[gBattlerTarget].hp != 0
-             && (Random() % 3) == 0
+             && RandomWeighted(RNG_CUTE_CHARM, 2, 1)
              && GetBattlerAbility(gBattlerAttacker) != ABILITY_OBLIVIOUS
              && !IsAbilityOnSide(gBattlerAttacker, ABILITY_AROMA_VEIL)
              && GetGenderFromSpeciesAndPersonality(speciesAtk, pidAtk) != GetGenderFromSpeciesAndPersonality(speciesDef, pidDef)
@@ -5915,7 +5915,7 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u16 ability, u8 special, u16 move
             if (!(gMoveResultFlags & MOVE_RESULT_NO_EFFECT)
              && gBattleMons[gBattlerTarget].hp != 0
              && !gProtectStructs[gBattlerAttacker].confusionSelfDmg
-             && (Random() % 10) == 0
+             && RandomWeighted(RNG_STENCH, 9, 1)
              && !IS_MOVE_STATUS(move)
              && !sMovesNotAffectedByStench[gCurrentMove])
             {
@@ -7635,7 +7635,7 @@ u8 ItemBattleEffects(u8 caseID, u8 battlerId, bool8 moveTurn)
             if (gBattleMoveDamage != 0  // Need to have done damage
                 && !(gMoveResultFlags & MOVE_RESULT_NO_EFFECT)
                 && TARGET_TURN_DAMAGED
-                && (Random() % 100) < atkHoldEffectParam
+                && RandomPercentage(RNG_HOLD_EFFECT_FLINCH, atkHoldEffectParam)
                 && gBattleMoves[gCurrentMove].flags & FLAG_KINGS_ROCK_AFFECTED
                 && gBattleMons[gBattlerTarget].hp)
             {
@@ -9756,7 +9756,7 @@ static s32 DoMoveDamageCalc(u16 move, u8 battlerAtk, u8 battlerDef, u8 moveType,
     // Add a random factor.
     if (randomFactor)
     {
-        dmg *= 100 - (Random() % 16);
+        dmg *= 100 - RandomUniform(RNG_DAMAGE_MODIFIER, 0, 15);
         dmg /= 100;
     }
 
diff --git a/src/random.c b/src/random.c
index de923fba6..e59b56387 100644
--- a/src/random.c
+++ b/src/random.c
@@ -31,3 +31,27 @@ u16 Random2(void)
     gRng2Value = ISO_RANDOMIZE1(gRng2Value);
     return gRng2Value >> 16;
 }
+
+__attribute__((weak, alias("RandomUniformDefault")))
+u32 RandomUniform(enum RandomTag tag, u32 lo, u32 hi);
+
+__attribute__((weak, alias("RandomWeightedArrayDefault")))
+u32 RandomWeightedArray(enum RandomTag tag, u32 sum, u32 n, const u8 *weights);
+
+u32 RandomUniformDefault(enum RandomTag tag, u32 lo, u32 hi)
+{
+    return lo + (((hi - lo) * Random()) >> 16);
+}
+
+u32 RandomWeightedArrayDefault(enum RandomTag tag, u32 sum, u32 n, const u8 *weights)
+{
+    s32 i, targetSum;
+    targetSum = (sum * Random()) >> 16;
+    for (i = 0; i < n - 1; i++)
+    {
+        targetSum -= weights[i];
+        if (targetSum < 0)
+            return i;
+    }
+    return n - 1;
+}
diff --git a/test/ability_compound_eyes.c b/test/ability_compound_eyes.c
index 97ab84dd2..bbe718176 100644
--- a/test/ability_compound_eyes.c
+++ b/test/ability_compound_eyes.c
@@ -3,7 +3,7 @@
 
 SINGLE_BATTLE_TEST("Compound Eyes raises accuracy")
 {
-    PASSES_RANDOMLY(91, 100);
+    PASSES_RANDOMLY(91, 100, RNG_ACCURACY);
     GIVEN {
         ASSUME(gBattleMoves[MOVE_THUNDER].accuracy == 70);
         PLAYER(SPECIES_BUTTERFREE) { Ability(ABILITY_COMPOUND_EYES); };
@@ -22,7 +22,7 @@ SINGLE_BATTLE_TEST("Compound Eyes raises accuracy")
 SINGLE_BATTLE_TEST("Compound Eyes does not affect OHKO moves")
 {
     KNOWN_FAILING;
-    PASSES_RANDOMLY(30, 100);
+    PASSES_RANDOMLY(30, 100, RNG_ACCURACY);
     GIVEN {
         ASSUME(gBattleMoves[MOVE_FISSURE].accuracy == 30);
         ASSUME(gBattleMoves[MOVE_FISSURE].effect == EFFECT_OHKO);
diff --git a/test/ability_cute_charm.c b/test/ability_cute_charm.c
index 5e089efdf..ccf5c490b 100644
--- a/test/ability_cute_charm.c
+++ b/test/ability_cute_charm.c
@@ -1,9 +1,6 @@
 #include "global.h"
 #include "test_battle.h"
 
-// TODO: Currently PASSES_RANDOMLY is incapable of testing Cute Charm
-// because it only activates 33% of the time, but we only want to
-// measure the 50% of the time that the infatuation prevents our move.
 SINGLE_BATTLE_TEST("Cute Charm inflicts infatuation on contact")
 {
     u32 move;
diff --git a/test/ability_sand_veil.c b/test/ability_sand_veil.c
index 5d2325140..5514f27c0 100644
--- a/test/ability_sand_veil.c
+++ b/test/ability_sand_veil.c
@@ -16,7 +16,7 @@ SINGLE_BATTLE_TEST("Sand Veil prevents damage from sandstorm")
 
 SINGLE_BATTLE_TEST("Sand Veil reduces accuracy during sandstorm")
 {
-    PASSES_RANDOMLY(4,5);
+    PASSES_RANDOMLY(4, 5, RNG_ACCURACY);
     GIVEN {
         ASSUME(gBattleMoves[MOVE_POUND].accuracy == 100);
         PLAYER(SPECIES_SANDSHREW) { Ability(ABILITY_SAND_VEIL); };
diff --git a/test/ability_stench.c b/test/ability_stench.c
index 18d909768..fb76ebc07 100644
--- a/test/ability_stench.c
+++ b/test/ability_stench.c
@@ -3,7 +3,7 @@
 
 SINGLE_BATTLE_TEST("Stench has a 10% chance to flinch")
 {
-    PASSES_RANDOMLY(1,10);
+    PASSES_RANDOMLY(1, 10, RNG_STENCH);
     GIVEN {
         ASSUME(gBattleMoves[MOVE_TACKLE].power > 0);
         PLAYER(SPECIES_GRIMER) { Ability(ABILITY_STENCH); };
@@ -17,7 +17,8 @@ SINGLE_BATTLE_TEST("Stench has a 10% chance to flinch")
 
 SINGLE_BATTLE_TEST("Stench does not stack with King's Rock")
 {
-    PASSES_RANDOMLY(1,10);
+    KNOWN_FAILING;
+    PASSES_RANDOMLY(1, 10, RNG_STENCH);
     GIVEN {
         ASSUME(gItems[ITEM_KINGS_ROCK].holdEffect == HOLD_EFFECT_FLINCH);
         ASSUME(gBattleMoves[MOVE_TACKLE].power > 0);
diff --git a/test/hold_effect_red_card.c b/test/hold_effect_red_card.c
index 882007cfd..0a8bb3460 100644
--- a/test/hold_effect_red_card.c
+++ b/test/hold_effect_red_card.c
@@ -8,7 +8,7 @@ ASSUMPTIONS
 
 SINGLE_BATTLE_TEST("Red Card switches the attacker with a random non-fainted replacement")
 {
-    PASSES_RANDOMLY(1, 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET) { Item(ITEM_RED_CARD); }
         OPPONENT(SPECIES_WOBBUFFET);
@@ -27,7 +27,7 @@ SINGLE_BATTLE_TEST("Red Card switches the attacker with a random non-fainted rep
 
 DOUBLE_BATTLE_TEST("Red Card switches the target with a random non-battler, non-fainted replacement")
 {
-    PASSES_RANDOMLY(1, 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET) { Item(ITEM_RED_CARD); }
         PLAYER(SPECIES_WYNAUT);
diff --git a/test/move.c b/test/move.c
index d7e759f27..9ad336330 100644
--- a/test/move.c
+++ b/test/move.c
@@ -10,7 +10,7 @@ SINGLE_BATTLE_TEST("Accuracy controls the proportion of misses")
     PARAMETRIZE { move = MOVE_RAZOR_LEAF; }
     PARAMETRIZE { move = MOVE_SCRATCH; }
     ASSUME(0 < gBattleMoves[move].accuracy && gBattleMoves[move].accuracy <= 100);
-    PASSES_RANDOMLY(gBattleMoves[move].accuracy, 100);
+    PASSES_RANDOMLY(gBattleMoves[move].accuracy, 100, RNG_ACCURACY);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
@@ -27,10 +27,9 @@ SINGLE_BATTLE_TEST("Secondary Effect Chance controls the proportion of secondary
     PARAMETRIZE { move = MOVE_THUNDER_SHOCK; }
     PARAMETRIZE { move = MOVE_DISCHARGE; }
     PARAMETRIZE { move = MOVE_NUZZLE; }
-    ASSUME(gBattleMoves[move].accuracy == 100);
     ASSUME(gBattleMoves[move].effect == EFFECT_PARALYZE_HIT);
     ASSUME(0 < gBattleMoves[move].secondaryEffectChance && gBattleMoves[move].secondaryEffectChance <= 100);
-    PASSES_RANDOMLY(gBattleMoves[move].secondaryEffectChance, 100);
+    PASSES_RANDOMLY(gBattleMoves[move].secondaryEffectChance, 100, RNG_SECONDARY_EFFECT);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
@@ -70,6 +69,7 @@ SINGLE_BATTLE_TEST("Turn order is determined by speed if priority ties")
 
 SINGLE_BATTLE_TEST("Turn order is determined randomly if priority and speed tie")
 {
+    KNOWN_FAILING; // The algorithm is significantly biased.
     PASSES_RANDOMLY(1, 2);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET) { Speed(1); }
@@ -85,15 +85,29 @@ SINGLE_BATTLE_TEST("Turn order is determined randomly if priority and speed tie"
 SINGLE_BATTLE_TEST("Critical hits occur at a 1/24 rate")
 {
     ASSUME(B_CRIT_CHANCE >= GEN_7);
-    ASSUME(gBattleMoves[MOVE_SCRATCH].accuracy == 100);
-    PASSES_RANDOMLY(100 / 24, 100);
+    PASSES_RANDOMLY(1, 24, RNG_CRITICAL_HIT);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
         TURN { MOVE(player, MOVE_SCRATCH); }
     } SCENE {
-        MESSAGE("It's a critical hit!");
+        MESSAGE("A critical hit!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Slash's critical hits occur at a 1/8 rate")
+{
+    ASSUME(B_CRIT_CHANCE >= GEN_7);
+    ASSUME(gBattleMoves[MOVE_SLASH].flags & FLAG_HIGH_CRIT);
+    PASSES_RANDOMLY(1, 8, RNG_CRITICAL_HIT);
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET);
+    } WHEN {
+        TURN { MOVE(player, MOVE_SLASH); }
+    } SCENE {
+        MESSAGE("A critical hit!");
     }
 }
 
diff --git a/test/move_effect_accuracy_down.c b/test/move_effect_accuracy_down.c
index 2a90d8ea2..a6a79d8db 100644
--- a/test/move_effect_accuracy_down.c
+++ b/test/move_effect_accuracy_down.c
@@ -9,7 +9,7 @@ ASSUMPTIONS
 SINGLE_BATTLE_TEST("Sand Attack lowers Accuracy")
 {
     ASSUME(gBattleMoves[MOVE_SCRATCH].accuracy == 100);
-    PASSES_RANDOMLY(gBattleMoves[MOVE_SCRATCH].accuracy * 3 / 4, 100);
+    PASSES_RANDOMLY(gBattleMoves[MOVE_SCRATCH].accuracy * 3 / 4, 100, RNG_ACCURACY);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
diff --git a/test/move_effect_defog.c b/test/move_effect_defog.c
index d877899bb..e9e55d57f 100644
--- a/test/move_effect_defog.c
+++ b/test/move_effect_defog.c
@@ -327,7 +327,6 @@ DOUBLE_BATTLE_TEST("Defog lowers evasiveness by 1 and removes Aurora Veil from p
 
 DOUBLE_BATTLE_TEST("Defog lowers evasiveness by 1 and removes everything it can")
 {
-    bool32 defogTurn = FALSE;
     GIVEN {
         ASSUME(gBattleMoves[MOVE_HAIL].effect == EFFECT_HAIL);
         ASSUME(gSpeciesInfo[SPECIES_GLALIE].types[0] == TYPE_ICE);
@@ -343,33 +342,29 @@ DOUBLE_BATTLE_TEST("Defog lowers evasiveness by 1 and removes everything it can"
         TURN { MOVE(playerLeft, MOVE_STICKY_WEB); MOVE(playerRight, MOVE_SPIKES); MOVE(opponentLeft, MOVE_STICKY_WEB); MOVE(opponentRight, MOVE_SPIKES); }
         TURN { SWITCH(playerLeft, 2); SWITCH(playerRight, 3); SWITCH(opponentLeft, 2); SWITCH(opponentRight, 3);}
         TURN { MOVE(playerLeft, MOVE_TOXIC_SPIKES); MOVE(playerRight, MOVE_STEALTH_ROCK); MOVE(opponentLeft, MOVE_TOXIC_SPIKES); MOVE(opponentRight, MOVE_STEALTH_ROCK); }
-        TURN { MOVE(playerLeft, MOVE_HAIL); MOVE(playerRight, MOVE_AURORA_VEIL); MOVE(opponentLeft, MOVE_AURORA_VEIL); MOVE(opponentRight, MOVE_STEALTH_ROCK); }
-        TURN { MOVE(playerLeft, MOVE_REFLECT); MOVE(playerRight, MOVE_LIGHT_SCREEN); MOVE(opponentLeft, MOVE_REFLECT); MOVE(opponentRight, MOVE_LIGHT_SCREEN); }
-        TURN { MOVE(playerLeft, MOVE_MIST); MOVE(playerRight, MOVE_SAFEGUARD); MOVE(opponentLeft, MOVE_MIST); MOVE(opponentRight, MOVE_SAFEGUARD); }
-        TURN { defogTurn = TRUE ; MOVE(opponentRight, MOVE_DEFOG, target:playerLeft);}
+        TURN { MOVE(playerLeft, MOVE_HAIL); MOVE(playerRight, MOVE_AURORA_VEIL); MOVE(opponentLeft, MOVE_AURORA_VEIL); MOVE(opponentRight, MOVE_LIGHT_SCREEN); }
+        TURN { MOVE(playerLeft, MOVE_REFLECT); MOVE(playerRight, MOVE_LIGHT_SCREEN); MOVE(opponentLeft, MOVE_REFLECT); MOVE(opponentRight, MOVE_SAFEGUARD); }
+        TURN { MOVE(playerLeft, MOVE_MIST); MOVE(playerRight, MOVE_SAFEGUARD); MOVE(opponentLeft, MOVE_MIST); MOVE(opponentRight, MOVE_DEFOG, target: playerLeft); }
     } SCENE {
-        if (defogTurn == TRUE)
-        {
-            MESSAGE("Foe Glalie used Defog!");
-            MESSAGE("Glalie is protected by MIST!");
-            ANIMATION(ANIM_TYPE_MOVE, MOVE_DEFOG, opponentRight);
-            // Player side
-            MESSAGE("Ally's Reflect wore off!");
-            MESSAGE("Ally's Light Screen wore off!");
-            MESSAGE("Ally's Mist wore off!");
-            MESSAGE("Ally's Aurora Veil wore off!");
-            MESSAGE("Ally's Safeguard wore off!");
+        MESSAGE("Foe Glalie used Defog!");
+        MESSAGE("Glalie is protected by MIST!");
+        ANIMATION(ANIM_TYPE_MOVE, MOVE_DEFOG, opponentRight);
+        // Player side
+        MESSAGE("Ally's Reflect wore off!");
+        MESSAGE("Ally's Light Screen wore off!");
+        MESSAGE("Ally's Mist wore off!");
+        MESSAGE("Ally's Aurora Veil wore off!");
+        MESSAGE("Ally's Safeguard wore off!");
 
-            MESSAGE("The spikes disappeared from the ground around your team!");
-            MESSAGE("The pointed stones disappeared from around your team!");
-            MESSAGE("The poison spikes disappeared from the ground around your team!");
-            MESSAGE("The sticky web has disappeared from the ground around your team!");
+        MESSAGE("The spikes disappeared from the ground around your team!");
+        MESSAGE("The pointed stones disappeared from around your team!");
+        MESSAGE("The poison spikes disappeared from the ground around your team!");
+        MESSAGE("The sticky web has disappeared from the ground around your team!");
 
-            // Opponent side
-            MESSAGE("The spikes disappeared from the ground around the opposing team!");
-            MESSAGE("The pointed stones disappeared from around the opposing team!");
-            MESSAGE("The poison spikes disappeared from the ground around the opposing team!");
-            MESSAGE("The sticky web has disappeared from the ground around the opposing team!");
-        }
+        // Opponent side
+        MESSAGE("The spikes disappeared from the ground around the opposing team!");
+        MESSAGE("The pointed stones disappeared from around the opposing team!");
+        MESSAGE("The poison spikes disappeared from the ground around the opposing team!");
+        MESSAGE("The sticky web has disappeared from the ground around the opposing team!");
     }
 }
diff --git a/test/move_effect_evasion_up.c b/test/move_effect_evasion_up.c
index d14d15334..4a4e99db7 100644
--- a/test/move_effect_evasion_up.c
+++ b/test/move_effect_evasion_up.c
@@ -9,7 +9,7 @@ ASSUMPTIONS
 SINGLE_BATTLE_TEST("Double Team raises Evasion")
 {
     ASSUME(gBattleMoves[MOVE_SCRATCH].accuracy == 100);
-    PASSES_RANDOMLY(gBattleMoves[MOVE_SCRATCH].accuracy * 3 / 4, 100);
+    PASSES_RANDOMLY(gBattleMoves[MOVE_SCRATCH].accuracy * 3 / 4, 100, RNG_ACCURACY);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
diff --git a/test/move_effect_hit_switch_target.c b/test/move_effect_hit_switch_target.c
index 9c50a4e4c..5af3062a4 100644
--- a/test/move_effect_hit_switch_target.c
+++ b/test/move_effect_hit_switch_target.c
@@ -9,8 +9,7 @@ ASSUMPTIONS
 
 SINGLE_BATTLE_TEST("Dragon Tail switches the target with a random non-fainted replacement")
 {
-    KNOWN_FAILING; // Only 18/50. Waiting for an improved PASSES_RANDOMLY.
-    PASSES_RANDOMLY(90 * 1, 100 * 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
@@ -27,7 +26,7 @@ SINGLE_BATTLE_TEST("Dragon Tail switches the target with a random non-fainted re
 
 DOUBLE_BATTLE_TEST("Dragon Tail switches the target with a random non-battler, non-fainted replacement")
 {
-    PASSES_RANDOMLY(90 * 1, 100 * 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         PLAYER(SPECIES_WYNAUT);
diff --git a/test/move_effect_rampage.c b/test/move_effect_rampage.c
index a3afebf6b..aa4a002fb 100644
--- a/test/move_effect_rampage.c
+++ b/test/move_effect_rampage.c
@@ -8,7 +8,7 @@ ASSUMPTIONS
 
 SINGLE_BATTLE_TEST("Thrash lasts for 2 or 3 turns")
 {
-    PASSES_RANDOMLY(1, 2);
+    PASSES_RANDOMLY(1, 2, RNG_RAMPAGE_TURNS);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
@@ -26,7 +26,6 @@ SINGLE_BATTLE_TEST("Thrash lasts for 2 or 3 turns")
 SINGLE_BATTLE_TEST("Thrash confuses the user after it finishes")
 {
     GIVEN {
-        RNGSeed(0x00000000);
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
@@ -45,7 +44,6 @@ SINGLE_BATTLE_TEST("Thrash does not confuse the user if it is canceled on turn 1
 {
     GIVEN {
         ASSUME(B_RAMPAGE_CANCELLING >= GEN_5);
-        RNGSeed(0x00000000);
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
@@ -61,7 +59,6 @@ SINGLE_BATTLE_TEST("Thrash does not confuse the user if it is canceled on turn 2
 {
     GIVEN {
         ASSUME(B_RAMPAGE_CANCELLING >= GEN_5);
-        RNGSeed(0x00000000);
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
@@ -78,7 +75,6 @@ SINGLE_BATTLE_TEST("Thrash confuses the user if it is canceled on turn 3 of 3")
     KNOWN_FAILING;
     GIVEN {
         ASSUME(B_RAMPAGE_CANCELLING >= GEN_5);
-        RNGSeed(0x00000000);
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
diff --git a/test/move_effect_roar.c b/test/move_effect_roar.c
index 2d4eadda8..99256b298 100644
--- a/test/move_effect_roar.c
+++ b/test/move_effect_roar.c
@@ -8,7 +8,7 @@ ASSUMPTIONS
 
 SINGLE_BATTLE_TEST("Roar switches the target with a random non-fainted replacement")
 {
-    PASSES_RANDOMLY(1, 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
@@ -25,7 +25,7 @@ SINGLE_BATTLE_TEST("Roar switches the target with a random non-fainted replaceme
 
 DOUBLE_BATTLE_TEST("Roar switches the target with a random non-battler, non-fainted replacement")
 {
-    PASSES_RANDOMLY(1, 2);
+    PASSES_RANDOMLY(1, 2, RNG_FORCE_RANDOM_SWITCH);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         PLAYER(SPECIES_WYNAUT);
diff --git a/test/move_effect_sleep.c b/test/move_effect_sleep.c
index c80faf4bd..c34e1248e 100644
--- a/test/move_effect_sleep.c
+++ b/test/move_effect_sleep.c
@@ -6,16 +6,33 @@ ASSUMPTIONS
     ASSUME(gBattleMoves[MOVE_HYPNOSIS].effect == EFFECT_SLEEP);
 }
 
-SINGLE_BATTLE_TEST("Hypnosis inflicts sleep")
+SINGLE_BATTLE_TEST("Hypnosis inflicts 1-3 turns of sleep")
 {
+    u32 turns, count;
+    ASSUME(B_SLEEP_TURNS >= GEN_5);
+    PARAMETRIZE { turns = 1; }
+    PARAMETRIZE { turns = 2; }
+    PARAMETRIZE { turns = 3; }
+    PASSES_RANDOMLY(1, 3, RNG_SLEEP_TURNS);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET);
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
-        TURN { MOVE(player, MOVE_HYPNOSIS); }
+        TURN { MOVE(player, MOVE_HYPNOSIS); MOVE(opponent, MOVE_CELEBRATE); }
+        for (count = 0; count < turns; ++count)
+            TURN {}
     } SCENE {
         ANIMATION(ANIM_TYPE_MOVE, MOVE_HYPNOSIS, player);
         ANIMATION(ANIM_TYPE_STATUS, B_ANIM_STATUS_SLP, opponent);
+        MESSAGE("Foe Wobbuffet fell asleep!");
         STATUS_ICON(opponent, sleep: TRUE);
+        for (count = 0; count < turns; ++count)
+        {
+            if (count < turns - 1)
+                MESSAGE("Foe Wobbuffet is fast asleep.");
+            ANIMATION(ANIM_TYPE_STATUS, B_ANIM_STATUS_SLP, opponent);
+        }
+        MESSAGE("Foe Wobbuffet woke up!");
+        STATUS_ICON(opponent, none: TRUE);
     }
 }
diff --git a/test/random.c b/test/random.c
new file mode 100644
index 000000000..abaf4a4f8
--- /dev/null
+++ b/test/random.c
@@ -0,0 +1,81 @@
+#include "global.h"
+#include "test.h"
+#include "random.h"
+
+TEST("RandomUniform generates lo..hi")
+{
+    u32 lo, hi, i;
+    PARAMETRIZE { lo = 0; hi = 1; }
+    PARAMETRIZE { lo = 0; hi = 2; }
+    PARAMETRIZE { lo = 0; hi = 3; }
+    PARAMETRIZE { lo = 2; hi = 4; }
+    for (i = 0; i < 1024; i++)
+    {
+        u32 r = RandomUniformDefault(RNG_NONE, lo, hi);
+        EXPECT(lo <= r && r <= hi);
+    }
+}
+
+TEST("RandomWeighted generates 0..n-1")
+{
+    u32 n, sum, i;
+    static const u8 ws[8] = { 1, 1, 1, 1, 1, 1, 1, 1 };
+    PARAMETRIZE { n = 1; }
+    PARAMETRIZE { n = 2; }
+    PARAMETRIZE { n = 3; }
+    PARAMETRIZE { n = 4; }
+    ASSUME(n <= ARRAY_COUNT(ws));
+    for (i = 0, sum = 0; i < n; i++)
+        sum += ws[i];
+    for (i = 0; i < 1024; i++)
+    {
+        u32 r = RandomWeightedArrayDefault(RNG_NONE, sum, n, ws);
+        EXPECT(0 <= r && r < n);
+    }
+}
+
+TEST("RandomUniform generates uniform distribution")
+{
+    u32 i, error;
+    u16 distribution[4];
+
+    memset(distribution, 0, sizeof(distribution));
+    for (i = 0; i < 4096; i++)
+    {
+        u32 r = RandomUniformDefault(RNG_NONE, 0, ARRAY_COUNT(distribution));
+        EXPECT(0 <= r && r < ARRAY_COUNT(distribution));
+        distribution[r]++;
+    }
+
+    error = 0;
+    for (i = 0; i < ARRAY_COUNT(distribution); i++)
+        error += abs(UQ_4_12(0.25) - distribution[i]);
+
+    EXPECT_LT(error, UQ_4_12(0.025));
+}
+
+TEST("RandomWeighted generates distribution in proportion to the weights")
+{
+    u32 i, sum, error;
+    static const u8 ws[4] = { 1, 2, 2, 3 };
+    u16 distribution[ARRAY_COUNT(ws)];
+
+    for (i = 0, sum = 0; i < ARRAY_COUNT(ws); i++)
+        sum += ws[i];
+
+    memset(distribution, 0, sizeof(distribution));
+    for (i = 0; i < 4096; i++)
+    {
+        u32 r = RandomWeightedArrayDefault(RNG_NONE, sum, ARRAY_COUNT(ws), ws);
+        EXPECT(0 <= r && r < ARRAY_COUNT(ws));
+        distribution[r]++;
+    }
+
+    error = 0;
+    error += abs(UQ_4_12(0.125) - distribution[0]);
+    error += abs(UQ_4_12(0.250) - distribution[1]);
+    error += abs(UQ_4_12(0.250) - distribution[2]);
+    error += abs(UQ_4_12(0.375) - distribution[3]);
+
+    EXPECT_LT(error, UQ_4_12(0.025));
+}
diff --git a/test/status1.c b/test/status1.c
index 63a9cd041..f4d3c7d94 100644
--- a/test/status1.c
+++ b/test/status1.c
@@ -72,7 +72,7 @@ SINGLE_BATTLE_TEST("Burn reduces attack by 50%", s16 damage)
 
 SINGLE_BATTLE_TEST("Freeze has a 20% chance of being thawed")
 {
-    PASSES_RANDOMLY(20, 100);
+    PASSES_RANDOMLY(20, 100, RNG_FROZEN);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET) { Status1(STATUS1_FREEZE); }
         OPPONENT(SPECIES_WOBBUFFET);
@@ -90,9 +90,8 @@ SINGLE_BATTLE_TEST("Freeze is thawed by opponent's Fire-type attacks")
         PLAYER(SPECIES_WOBBUFFET) { Status1(STATUS1_FREEZE); }
         OPPONENT(SPECIES_WOBBUFFET);
     } WHEN {
-        TURN { MOVE(player, MOVE_CELEBRATE); MOVE(opponent, MOVE_EMBER); }
+        TURN { MOVE(opponent, MOVE_EMBER); MOVE(player, MOVE_CELEBRATE); }
     } SCENE {
-        MESSAGE("Wobbuffet is frozen solid!");
         MESSAGE("Foe Wobbuffet used Ember!");
         MESSAGE("Wobbuffet was defrosted!");
         STATUS_ICON(player, none: TRUE);
@@ -145,7 +144,7 @@ SINGLE_BATTLE_TEST("Paralysis reduces speed by 50%")
 
 SINGLE_BATTLE_TEST("Paralysis has a 25% chance of skipping the turn")
 {
-    PASSES_RANDOMLY(25, 100);
+    PASSES_RANDOMLY(25, 100, RNG_PARALYSIS);
     GIVEN {
         PLAYER(SPECIES_WOBBUFFET) { Status1(STATUS1_PARALYSIS); }
         OPPONENT(SPECIES_WOBBUFFET);
diff --git a/test/test.h b/test/test.h
index 27475bb48..764443302 100644
--- a/test/test.h
+++ b/test/test.h
@@ -53,6 +53,16 @@ extern const u8 gTestRunnerI;
 extern const char gTestRunnerArgv[256];
 
 extern const struct TestRunner gAssumptionsRunner;
+
+struct FunctionTestRunnerState
+{
+    u8 parameters;
+    u8 runParameter;
+};
+
+extern const struct TestRunner gFunctionTestRunner;
+extern struct FunctionTestRunnerState *gFunctionTestRunnerState;
+
 extern struct TestRunnerState gTestRunnerState;
 
 void CB2_TestRunner(void);
@@ -62,6 +72,17 @@ void Test_ExitWithResult(enum TestResult, const char *fmt, ...);
 
 s32 MgbaPrintf_(const char *fmt, ...);
 
+#define TEST(_name) \
+    static void CAT(Test, __LINE__)(void); \
+    __attribute__((section(".tests"))) static const struct Test CAT(sTest, __LINE__) = \
+    { \
+        .name = _name, \
+        .filename = __FILE__, \
+        .runner = &gFunctionTestRunner, \
+        .data = (void *)CAT(Test, __LINE__), \
+    }; \
+    static void CAT(Test, __LINE__)(void)
+
 #define ASSUMPTIONS \
     static void Assumptions(void); \
     __attribute__((section(".tests"))) static const struct Test sAssumptions = \
@@ -138,4 +159,6 @@ s32 MgbaPrintf_(const char *fmt, ...);
 #define KNOWN_FAILING \
     Test_ExpectedResult(TEST_RESULT_FAIL)
 
+#define PARAMETRIZE if (gFunctionTestRunnerState->parameters++ == gFunctionTestRunnerState->runParameter)
+
 #endif
diff --git a/test/test_battle.h b/test/test_battle.h
index 16918251b..8c554216e 100644
--- a/test/test_battle.h
+++ b/test/test_battle.h
@@ -65,8 +65,9 @@
  * single turn. MOVE causes the player to use Stun Spore and adds the
  * move to the Pokémon's moveset if an explicit Moves was not specified.
  * Pokémon that are not mentioned in a TURN use Celebrate.
- * The test runner attempts to rig the RNG so that the first move used
- * in a turn does not miss and activates its secondary effects (if any).
+ * The test runner rigs the RNG so that unless otherwise specified,
+ * moves always hit, never critical hit, always activate their secondary
+ * effects, and always roll the same damage modifier.
  *
  * SCENE describes the player-visible output of the battle. In this case
  * ANIMATION checks that the Stun Spore animation played, MESSAGE checks
@@ -228,12 +229,35 @@
  *         }
  *     }
  *
- * PASSES_RANDOMLY(successes, trials)
- * Checks that the test passes approximately successes/trials. Used for
- * testing RNG-based attacks, e.g.:
+ * PASSES_RANDOMLY(successes, trials, [tag])
+ * Checks that the test passes successes/trials. If tag is provided, the
+ * test is run for each value that the tag can produce. For example, to
+ * check that Paralysis causes the turn to be skipped 25/100 times, we
+ * can write the following test that passes only if the Pokémon is fully
+ * paralyzed and specify that we expect it to pass 25/100 times when
+ * RNG_PARALYSIS varies:
+ *     SINGLE_BATTLE_TEST("Paralysis has a 25% chance of skipping the turn")
+ *     {
+ *         PASSES_RANDOMLY(25, 100, RNG_PARALYSIS);
+ *         GIVEN {
+ *             PLAYER(SPECIES_WOBBUFFET) { Status1(STATUS1_PARALYSIS); }
+ *             OPPONENT(SPECIES_WOBBUFFET);
+ *         } WHEN {
+ *             TURN { MOVE(player, MOVE_CELEBRATE); }
+ *         } SCENE {
+ *             MESSAGE("Wobbuffet is paralyzed! It can't move!");
+ *         }
+ *     }
+ * All BattleRandom calls involving tag will return the same number, so
+ * this cannot be used to have two moves independently hit or miss, for
+ * example.
+ *
+ * If the tag is not provided, runs the test 50 times and computes an
+ * approximate pass ratio.
  *     PASSES_RANDOMLY(gBattleMoves[move].accuracy, 100);
- * Note that PASSES_RANDOMLY makes the tests run very slowly and should
- * be avoided where possible.
+ * Note that this mode of PASSES_RANDOMLY makes the tests run very
+ * slowly and should be avoided where possible. If the mechanic you are
+ * testing is missing its tag, you should add it.
  *
  * GIVEN
  * Contains the initial state of the parties before the battle.
@@ -419,6 +443,7 @@
 #include "battle_anim.h"
 #include "data.h"
 #include "item.h"
+#include "random.h"
 #include "recorded_battle.h"
 #include "test.h"
 #include "util.h"
@@ -433,6 +458,7 @@
 // NOTE: If the stack is too small the test runner will probably crash
 // or loop.
 #define BATTLE_TEST_STACK_SIZE 1024
+#define MAX_TURNS 16
 #define MAX_QUEUED_EVENTS 25
 
 enum { BATTLE_TEST_SINGLES, BATTLE_TEST_DOUBLES };
@@ -512,6 +538,13 @@ struct QueuedEvent
     } as;
 };
 
+struct BattlerTurn
+{
+    u8 hit:2;
+    u8 criticalHit:2;
+    u8 secondaryEffect:2;
+};
+
 struct BattleTestData
 {
     u8 stack[BATTLE_TEST_STACK_SIZE];
@@ -533,14 +566,13 @@ struct BattleTestData
     u8 turns;
     u8 actionBattlers;
     u8 moveBattlers;
-    bool8 hasRNGActions:1;
 
     struct RecordedBattleSave recordedBattle;
     u8 battleRecordTypes[MAX_BATTLERS_COUNT][BATTLER_RECORD_SIZE];
     u8 battleRecordSourceLineOffsets[MAX_BATTLERS_COUNT][BATTLER_RECORD_SIZE];
     u16 recordIndexes[MAX_BATTLERS_COUNT];
+    struct BattlerTurn battleRecordTurns[MAX_TURNS][MAX_BATTLERS_COUNT];
     u8 lastActionTurn;
-    u8 nextRNGTurn;
 
     u8 queuedEventsCount;
     u8 queueGroupType;
@@ -555,11 +587,12 @@ struct BattleTestRunnerState
     u8 parametersCount; // Valid only in BattleTest_Setup.
     u8 parameters;
     u8 runParameter;
+    u16 rngTag;
     u8 trials;
-    u8 expectedPasses;
-    u8 observedPasses;
-    u8 skippedTrials;
     u8 runTrial;
+    u16 expectedRatio;
+    u16 observedRatio;
+    u16 trialRatio;
     bool8 runRandomly:1;
     bool8 runGiven:1;
     bool8 runWhen:1;
@@ -639,13 +672,20 @@ extern struct BattleTestRunnerState *gBattleTestRunnerState;
 
 /* Parametrize */
 
+#undef PARAMETRIZE // Override test/test.h's implementation.
+
 #define PARAMETRIZE if (gBattleTestRunnerState->parametersCount++ == i)
 
 /* Randomly */
 
-#define PASSES_RANDOMLY(passes, trials) for (; gBattleTestRunnerState->runRandomly; gBattleTestRunnerState->runRandomly = FALSE) Randomly(__LINE__, passes, trials)
+#define PASSES_RANDOMLY(passes, trials, ...) for (; gBattleTestRunnerState->runRandomly; gBattleTestRunnerState->runRandomly = FALSE) Randomly(__LINE__, passes, trials, (struct RandomlyContext) { __VA_ARGS__ })
 
-void Randomly(u32 sourceLine, u32 passes, u32 trials);
+struct RandomlyContext
+{
+    u16 tag;
+};
+
+void Randomly(u32 sourceLine, u32 passes, u32 trials, struct RandomlyContext);
 
 /* Given */
 
@@ -719,6 +759,8 @@ struct MoveContext
     u16 explicitHit:1;
     u16 criticalHit:1;
     u16 explicitCriticalHit:1;
+    u16 secondaryEffect:1;
+    u16 explicitSecondaryEffect:1;
     u16 megaEvolve:1;
     u16 explicitMegaEvolve:1;
     // TODO: u8 zMove:1;
diff --git a/test/test_runner.c b/test/test_runner.c
index f2d4d5de3..e3d286c55 100644
--- a/test/test_runner.c
+++ b/test/test_runner.c
@@ -4,6 +4,7 @@
 #include "gpu_regs.h"
 #include "main.h"
 #include "malloc.h"
+#include "random.h"
 #include "test.h"
 #include "test_runner.h"
 
@@ -12,6 +13,7 @@
 void CB2_TestRunner(void);
 
 EWRAM_DATA struct TestRunnerState gTestRunnerState;
+EWRAM_DATA struct FunctionTestRunnerState *gFunctionTestRunnerState;
 
 void TestRunner_Battle(const struct Test *);
 
@@ -230,6 +232,38 @@ void Test_ExpectedResult(enum TestResult result)
     gTestRunnerState.expectedResult = result;
 }
 
+static void FunctionTest_SetUp(void *data)
+{
+    (void)data;
+    gFunctionTestRunnerState = AllocZeroed(sizeof(*gFunctionTestRunnerState));
+    SeedRng(0);
+}
+
+static void FunctionTest_Run(void *data)
+{
+    void (*function)(void) = data;
+    do
+    {
+        if (gFunctionTestRunnerState->parameters)
+            MgbaPrintf_(":N%s %d/%d", gTestRunnerState.test->name, gFunctionTestRunnerState->runParameter + 1, gFunctionTestRunnerState->parameters);
+        gFunctionTestRunnerState->parameters = 0;
+        function();
+    } while (++gFunctionTestRunnerState->runParameter < gFunctionTestRunnerState->parameters);
+}
+
+static void FunctionTest_TearDown(void *data)
+{
+    (void)data;
+    FREE_AND_SET_NULL(gFunctionTestRunnerState);
+}
+
+const struct TestRunner gFunctionTestRunner =
+{
+    .setUp = FunctionTest_SetUp,
+    .run = FunctionTest_Run,
+    .tearDown = FunctionTest_TearDown,
+};
+
 static void Assumptions_Run(void *data)
 {
     void (*function)(void) = data;
@@ -288,11 +322,12 @@ static void Intr_Timer2(void)
 
 void Test_ExitWithResult(enum TestResult result, const char *fmt, ...)
 {
+    bool32 handled = FALSE;
     gTestRunnerState.result = result;
     ReinitCallbacks();
-    if (gTestRunnerState.test->runner->handleExitWithResult
-     && !gTestRunnerState.test->runner->handleExitWithResult(gTestRunnerState.test->data, result)
-     && gTestRunnerState.result != gTestRunnerState.expectedResult)
+    if (gTestRunnerState.test->runner->handleExitWithResult)
+        handled = gTestRunnerState.test->runner->handleExitWithResult(gTestRunnerState.test->data, result);
+    if (!handled && gTestRunnerState.result != gTestRunnerState.expectedResult)
     {
         va_list va;
         va_start(va, fmt);
diff --git a/test/test_runner_battle.c b/test/test_runner_battle.c
index f4b8aa4fd..d869bf2e5 100644
--- a/test/test_runner_battle.c
+++ b/test/test_runner_battle.c
@@ -15,26 +15,10 @@
 #define STATE gBattleTestRunnerState
 #define DATA gBattleTestRunnerState->data
 
-/* RNG seeds for controlling the first move of the turn.
- * Found via brute force. */
+#define RNG_SEED_DEFAULT 0x00000000
 
-/* Default seed, triggers most things.
- * The 1st roll % 100 is <= 29, to make 30%+ accuracycheck pass.
- * The 2nd roll is not a critical hit at the regular crit stage.
- * The 3rd roll is consumed by damagecalc.
- * The 4th roll is consumed by adjustdamage.
- * The 5th roll % 100 is <= 9, to make 10%+ seteffectwithchance pass
- *          and % 3 is == 0, to make Poison Point and other 1/3s pass. */
-#define RNG_SEED_DEFAULT 0x000002BE
-
-/* Causes the first attack to critical hit if B_CRIT_CHANCE >= GEN_6.
- * The 2nd roll % 24 == 0 to be a critical hit at any stage.
- * The other rolls match RNG_SEED_DEFAULT. */
-#define RNG_SEED_CRITICAL_HIT 0x0000A9F4
-
-/* Causes the first attack to miss if possible.
- * The 1st roll % 100 is 99, to make 99%- accuracycheck fail. */
-#define RNG_SEED_MISS 0x00000074
+#undef Q_4_12
+#define Q_4_12(n) (s32)((n) * 4096)
 
 EWRAM_DATA struct BattleTestRunnerState *gBattleTestRunnerState = NULL;
 
@@ -129,12 +113,13 @@ static u32 BattleTest_EstimateCost(void *data)
     if (!STATE)
         return 0;
     STATE->runRandomly = TRUE;
-    DATA.recordedBattle.rngSeed = RNG_SEED_DEFAULT;
     InvokeTestFunction(test);
     cost = 1;
     if (STATE->parametersCount != 0)
         cost *= STATE->parametersCount;
-    if (STATE->trials != 0)
+    if (STATE->trials == 1)
+        cost *= 3;
+    else if (STATE->trials > 1)
         cost *= STATE->trials;
     FREE_AND_SET_NULL(STATE);
     return cost;
@@ -162,6 +147,28 @@ static void BattleTest_SetUp(void *data)
     }
 }
 
+static void PrintTestName(void)
+{
+    if (STATE->trials && STATE->parameters)
+    {
+        if (STATE->trials == 1)
+            MgbaPrintf_(":N%s %d/%d (%d/?)", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters, STATE->runTrial + 1);
+        else
+            MgbaPrintf_(":N%s %d/%d (%d/%d)", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters, STATE->runTrial + 1, STATE->trials);
+    }
+    else if (STATE->trials)
+    {
+        if (STATE->trials == 1)
+            MgbaPrintf_(":N%s (%d/?)", gTestRunnerState.test->name, STATE->runTrial + 1);
+        else
+            MgbaPrintf_(":N%s (%d/%d)", gTestRunnerState.test->name, STATE->runTrial + 1, STATE->trials);
+    }
+    else if (STATE->parameters)
+    {
+        MgbaPrintf_(":N%s %d/%d", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters);
+    }
+}
+
 // This does not take into account priority, statuses, or any other
 // modifiers.
 static void SetImplicitSpeeds(void)
@@ -280,12 +287,82 @@ static void BattleTest_Run(void *data)
     STATE->checkProgressTrial = 0;
     STATE->checkProgressTurn = 0;
 
-    if (STATE->trials && STATE->parameters)
-        MgbaPrintf_(":N%s %d/%d (%d/%d)", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters, STATE->runTrial + 1, STATE->trials);
-    else if (STATE->trials)
-        MgbaPrintf_(":N%s (%d/%d)", gTestRunnerState.test->name, STATE->runTrial + 1, STATE->trials);
-    else if (STATE->parameters)
-        MgbaPrintf_(":N%s %d/%d", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters);
+    PrintTestName();
+}
+
+u32 RandomUniform(enum RandomTag tag, u32 lo, u32 hi)
+{
+    if (tag == STATE->rngTag)
+    {
+        u32 n = hi - lo + 1;
+        if (STATE->trials == 1)
+        {
+            STATE->trials = n;
+            PrintTestName();
+        }
+        else if (STATE->trials != n)
+        {
+            Test_ExitWithResult(TEST_RESULT_ERROR, "RandomUniform called with inconsistent trials %d and %d", STATE->trials, n);
+        }
+        STATE->trialRatio = Q_4_12(1) / n;
+        return STATE->runTrial + lo;
+    }
+
+    return hi;
+}
+
+u32 RandomWeightedArray(enum RandomTag tag, u32 sum, u32 n, const u8 *weights)
+{
+    const struct BattlerTurn *turn = NULL;
+    u32 default_ = n-1;
+
+    if (gCurrentTurnActionNumber < gBattlersCount)
+    {
+        u32 battlerId = gBattlerByTurnOrder[gCurrentTurnActionNumber];
+        turn = &DATA.battleRecordTurns[gBattleResults.battleTurnCounter][battlerId];
+    }
+
+    switch (tag)
+    {
+    case RNG_ACCURACY:
+        ASSUME(n == 2);
+        if (turn && turn->hit)
+            return turn->hit - 1;
+        default_ = TRUE;
+        break;
+
+    case RNG_CRITICAL_HIT:
+        ASSUME(n == 2);
+        if (turn && turn->criticalHit)
+            return turn->criticalHit - 1;
+        default_ = FALSE;
+        break;
+
+    case RNG_SECONDARY_EFFECT:
+        ASSUME(n == 2);
+        if (turn && turn->secondaryEffect)
+            return turn->secondaryEffect - 1;
+        default_ = TRUE;
+        break;
+    }
+
+    if (tag == STATE->rngTag)
+    {
+        if (STATE->trials == 1)
+        {
+            STATE->trials = n;
+            PrintTestName();
+        }
+        else if (STATE->trials != n)
+        {
+            Test_ExitWithResult(TEST_RESULT_ERROR, "RandomWeighted called with inconsistent trials %d and %d", STATE->trials, n);
+        }
+        // TODO: Detect inconsistent sum.
+        STATE->trialRatio = Q_4_12(weights[STATE->runTrial]) / sum;
+        return STATE->runTrial;
+    }
+
+    return default_;
 }
 
 static s32 TryAbilityPopUp(s32 i, s32 n, u32 battlerId, u32 ability)
@@ -711,42 +788,36 @@ static void CB2_BattleTest_NextTrial(void)
 
     SetMainCallback2(CB2_BattleTest_NextParameter);
 
+    switch (gTestRunnerState.result)
+    {
+    case TEST_RESULT_FAIL:
+        break;
+    case TEST_RESULT_PASS:
+        STATE->observedRatio += STATE->trialRatio;
+        break;
+    default:
+        return;
+    }
+    if (STATE->rngTag)
+        STATE->trialRatio = 0;
+
     if (++STATE->runTrial < STATE->trials)
     {
-        switch (gTestRunnerState.result)
-        {
-        case TEST_RESULT_FAIL:
-            break;
-        case TEST_RESULT_PASS:
-            STATE->observedPasses++;
-            break;
-        case TEST_RESULT_ASSUMPTION_FAIL:
-            STATE->skippedTrials++;
-            if (STATE->skippedTrials > STATE->trials / 4)
-                Test_ExitWithResult(TEST_RESULT_INVALID, "25%% of the trials were SKIPed");
-            break;
-        default:
-            return;
-        }
-        if (STATE->parameters)
-            MgbaPrintf_(":N%s %d/%d (%d/%d)", gTestRunnerState.test->name, STATE->runParameter + 1, STATE->parameters, STATE->runTrial + 1, STATE->trials);
-        else
-            MgbaPrintf_(":N%s (%d/%d)", gTestRunnerState.test->name, STATE->runTrial + 1, STATE->trials);
-                gTestRunnerState.result = TEST_RESULT_PASS;
+        PrintTestName();
+        gTestRunnerState.result = TEST_RESULT_PASS;
         DATA.recordedBattle.rngSeed = ISO_RANDOMIZE1(STATE->runTrial);
         DATA.queuedEvent = 0;
         DATA.lastActionTurn = 0;
-        DATA.nextRNGTurn = 0;
         SetVariablesForRecordedBattle(&DATA.recordedBattle);
         SetMainCallback2(CB2_InitBattle);
     }
     else
     {
-        // This is a tolerance of +/- 4%.
-        if (abs(STATE->observedPasses - STATE->expectedPasses) <= 2)
+        // This is a tolerance of +/- ~2%.
+        if (abs(STATE->observedRatio - STATE->expectedRatio) <= Q_4_12(0.02))
             gTestRunnerState.result = TEST_RESULT_PASS;
         else
-            Test_ExitWithResult(TEST_RESULT_FAIL, "Expected %d/%d passes, observed %d/%d", STATE->expectedPasses, STATE->trials, STATE->observedPasses, STATE->trials);
+            Test_ExitWithResult(TEST_RESULT_FAIL, "Expected %q passes/successes, observed %q", STATE->expectedRatio, STATE->observedRatio);
     }
 }
 
@@ -773,7 +844,8 @@ static bool32 BattleTest_CheckProgress(void *data)
 
 static bool32 BattleTest_HandleExitWithResult(void *data, enum TestResult result)
 {
-    if (result != TEST_RESULT_INVALID
+    if (result != TEST_RESULT_ASSUMPTION_FAIL
+     && result != TEST_RESULT_INVALID
      && result != TEST_RESULT_ERROR
      && result != TEST_RESULT_TIMEOUT
      && STATE->runTrial < STATE->trials)
@@ -787,16 +859,25 @@ static bool32 BattleTest_HandleExitWithResult(void *data, enum TestResult result
     }
 }
 
-void Randomly(u32 sourceLine, u32 passes, u32 trials)
+void Randomly(u32 sourceLine, u32 passes, u32 trials, struct RandomlyContext ctx)
 {
-    INVALID_IF(DATA.recordedBattle.rngSeed != RNG_SEED_DEFAULT, "RNG seed already set");
-    // This is a precision of 2%.
-    STATE->trials = 50;
-    STATE->expectedPasses = STATE->trials * passes / trials;
-    STATE->observedPasses = 0;
-    STATE->skippedTrials = 0;
+    INVALID_IF(passes > trials, "%d passes specified, but only %d trials", passes, trials);
+    STATE->rngTag = ctx.tag;
     STATE->runTrial = 0;
-    DATA.recordedBattle.rngSeed = 0;
+    STATE->expectedRatio = Q_4_12(passes) / trials;
+    STATE->observedRatio = 0;
+    if (STATE->rngTag)
+    {
+        STATE->trials = 1;
+        STATE->trialRatio = Q_4_12(1);
+    }
+    else
+    {
+        INVALID_IF(DATA.recordedBattle.rngSeed != RNG_SEED_DEFAULT, "RNG seed already set");
+        STATE->trials = 50;
+        STATE->trialRatio = Q_4_12(1) / STATE->trials;
+        DATA.recordedBattle.rngSeed = 0;
+    }
 }
 
 void RNGSeed_(u32 sourceLine, u32 seed)
@@ -1025,6 +1106,31 @@ void Status1_(u32 sourceLine, u32 status1)
     SetMonData(DATA.currentMon, MON_DATA_STATUS, &status1);
 }
 
+static const char *const sBattlerIdentifiersSingles[] =
+{
+    "player",
+    "opponent",
+};
+
+static const char *const sBattlerIdentifiersDoubles[] =
+{
+    "playerLeft",
+    "opponentLeft",
+    "playerRight",
+    "opponentRight",
+};
+
+static const char *BattlerIdentifier(s32 battlerId)
+{
+    const struct BattleTest *test = gTestRunnerState.test->data;
+    switch (test->type)
+    {
+    case BATTLE_TEST_SINGLES: return sBattlerIdentifiersSingles[battlerId];
+    case BATTLE_TEST_DOUBLES: return sBattlerIdentifiersDoubles[battlerId];
+    }
+    return "<unknown>";
+}
+
 static void PushBattlerAction(u32 sourceLine, s32 battlerId, u32 actionType, u32 byte)
 {
     u32 recordIndex = DATA.recordIndexes[battlerId]++;
@@ -1037,16 +1143,6 @@ static void PushBattlerAction(u32 sourceLine, s32 battlerId, u32 actionType, u32
 
 void BattleTest_CheckBattleRecordActionType(u32 battlerId, u32 recordIndex, u32 actionType)
 {
-    // TODO: Support explicit seeds for each turn?
-    if (DATA.nextRNGTurn == gBattleResults.battleTurnCounter
-     && (DATA.recordedBattle.rngSeed == RNG_SEED_DEFAULT
-      || DATA.recordedBattle.rngSeed == RNG_SEED_CRITICAL_HIT
-      || DATA.recordedBattle.rngSeed == RNG_SEED_MISS))
-    {
-        gRngValue = DATA.recordedBattle.rngSeed;
-        DATA.nextRNGTurn++;
-    }
-
     // An illegal move choice will cause the battle to request a new
     // move slot and target. This detects the move slot.
     if (actionType == RECORDED_MOVE_SLOT
@@ -1122,10 +1218,11 @@ void BattleTest_CheckBattleRecordActionType(u32 battlerId, u32 recordIndex, u32
 void OpenTurn(u32 sourceLine)
 {
     INVALID_IF(DATA.turnState != TURN_CLOSED, "Nested TURN");
+    if (DATA.turns == MAX_TURNS)
+        Test_ExitWithResult(TEST_RESULT_ERROR, "%s:%d: TURN exceeds MAX_TURNS", gTestRunnerState.test->filename, sourceLine);
     DATA.turnState = TURN_OPEN;
     DATA.actionBattlers = 0x00;
     DATA.moveBattlers = 0x00;
-    DATA.hasRNGActions = FALSE;
 }
 
 static void SetSlowerThan(s32 battlerId)
@@ -1195,7 +1292,6 @@ void Move(u32 sourceLine, struct BattlePokemon *battler, struct MoveContext ctx)
             else if (moveId == MOVE_NONE)
             {
                 INVALID_IF(DATA.explicitMoves[battlerId & BIT_SIDE] & (1 << DATA.currentMonIndexes[battlerId]), "Missing explicit %S", gMoveNames[ctx.move]);
-                INVALID_IF(i == MAX_MON_MOVES, "Too many different moves");
                 SetMonData(mon, MON_DATA_MOVE1 + i, &ctx.move);
                 SetMonData(DATA.currentMon, MON_DATA_PP1 + i, &gBattleMoves[ctx.move].pp);
                 moveSlot = i;
@@ -1203,6 +1299,7 @@ void Move(u32 sourceLine, struct BattlePokemon *battler, struct MoveContext ctx)
                 break;
             }
         }
+        INVALID_IF(i == MAX_MON_MOVES, "Too many different moves for %s", BattlerIdentifier(battlerId));
     }
     else if (ctx.explicitMoveSlot)
     {
@@ -1254,21 +1351,12 @@ void Move(u32 sourceLine, struct BattlePokemon *battler, struct MoveContext ctx)
         }
     }
 
-    if (ctx.explicitHit && !ctx.hit)
-    {
-        if (DATA.hasRNGActions != 0)
-            Test_ExitWithResult(TEST_RESULT_ERROR, "%s:%d: hit only supported on the first move", gTestRunnerState.test->filename, sourceLine);
-        INVALID_IF(DATA.recordedBattle.rngSeed != RNG_SEED_DEFAULT, "RNG seed already set");
-        DATA.recordedBattle.rngSeed = RNG_SEED_MISS;
-    }
-
-    if (ctx.explicitCriticalHit && ctx.criticalHit)
-    {
-        if (DATA.hasRNGActions != 0)
-            Test_ExitWithResult(TEST_RESULT_ERROR, "%s:%d: criticalHit only supported on the first move", gTestRunnerState.test->filename, sourceLine);
-        INVALID_IF(DATA.recordedBattle.rngSeed != RNG_SEED_DEFAULT, "RNG seed already set");
-        DATA.recordedBattle.rngSeed = RNG_SEED_CRITICAL_HIT;
-    }
+    if (ctx.explicitHit)
+        DATA.battleRecordTurns[DATA.turns][battlerId].hit = 1 + ctx.hit;
+    if (ctx.explicitCriticalHit)
+        DATA.battleRecordTurns[DATA.turns][battlerId].criticalHit = 1 + ctx.criticalHit;
+    if (ctx.explicitSecondaryEffect)
+        DATA.battleRecordTurns[DATA.turns][battlerId].secondaryEffect = 1 + ctx.secondaryEffect;
 
     if (!(DATA.actionBattlers & (1 << battlerId)))
     {
@@ -1289,14 +1377,6 @@ void Move(u32 sourceLine, struct BattlePokemon *battler, struct MoveContext ctx)
         DATA.actionBattlers |= 1 << battlerId;
         DATA.moveBattlers |= 1 << battlerId;
     }
-
-    // WARNING: Approximation. The move could still cause the RNG to
-    // advance.
-    if (gBattleMoves[moveId].accuracy != 0
-     || gBattleMoves[moveId].split != SPLIT_STATUS)
-    {
-        DATA.hasRNGActions = TRUE;
-    }
 }
 
 void ForcedMove(u32 sourceLine, struct BattlePokemon *battler)