diff --git a/core/src/Spawning.cpp b/core/src/Spawning.cpp
new file mode 100644
index 0000000..966592a
--- /dev/null
+++ b/core/src/Spawning.cpp
@@ -0,0 +1,160 @@
+#include "vf/Spawning.h"
+
+#include <algorithm>
+#include <cmath>
+
+#include "vf/Effect.h"
+
+namespace vf {
+
+namespace {
+
+constexpr int nightSkyLightPenalty = 11;
+
+float straightDistance(const Vec3& a, const Vec3& b) {
+    Vec3 delta = a - b;
+    return length(delta);
+}
+
+bool groundAndRoomFit(const SpawnRule& rule, const SpawnEnvironment& env, const Vec3i& cell) {
+    Block ground = env.world->blockAt(cell.x, cell.y - 1, cell.z);
+    if (!isSolid(ground)) return false;
+    if (!rule.allowsGround(ground)) return false;
+    return hasSpawnRoom(*env.world, cell, rule.requiredHeadroom);
+}
+
+bool lightFits(const SpawnRule& rule, const SpawnEnvironment& env, const Vec3i& cell) {
+    int blockLight = env.blockLightAt(cell);
+    int skyLight = env.skyLightAt(cell);
+    if (blockLight > rule.maxBlockLight) return false;
+    if (skyLight > rule.maxSkyLight) return false;
+    if (blockLight < rule.minBlockLight) return false;
+    if (skyLight < rule.minSkyLight) return false;
+    return true;
+}
+
+SpawnRule makeHostileRule(Species species, int packMin, int packMax) {
+    SpawnRule rule;
+    rule.species = species;
+    rule.category = SpawnCategory::Hostile;
+    rule.dimension = DimensionId::Overworld;
+    rule.maxBlockLight = 7;
+    rule.maxSkyLight = 7;
+    rule.minY = 1;
+    rule.maxY = 127;
+    rule.requiredHeadroom = 2;
+    rule.dayAllowed = false;
+    rule.nightAllowed = true;
+    rule.packMin = packMin;
+    rule.packMax = packMax;
+    rule.packSpread = 3;
+    rule.weight = 10;
+    rule.minPlayerDistance = 24.0f;
+    rule.maxPlayerDistance = 128.0f;
+    return rule;
+}
+
+SpawnRule makePassiveRule(Species species, int packMin, int packMax,
+                          std::vector<BiomeKind> biomeList) {
+    SpawnRule rule;
+    rule.species = species;
+    rule.category = SpawnCategory::Passive;
+    rule.dimension = DimensionId::Overworld;
+    rule.minBlockLight = 0;
+    rule.minSkyLight = 9;
+    rule.maxBlockLight = 15;
+    rule.maxSkyLight = 15;
+    rule.minY = 48;
+    rule.maxY = 127;
+    rule.groundBlocks = {Block::Grass};
+    rule.requiredHeadroom = 2;
+    rule.biomes = std::move(biomeList);
+    rule.dayAllowed = true;
+    rule.nightAllowed = true;
+    rule.packMin = packMin;
+    rule.packMax = packMax;
+    rule.packSpread = 4;
+    rule.weight = 12;
+    rule.minPlayerDistance = 12.0f;
+    rule.maxPlayerDistance = 96.0f;
+    return rule;
+}
+
+} // namespace
+
+const char* spawnCategoryName(SpawnCategory category) {
+    switch (category) {
+        case SpawnCategory::Hostile: return "hostile";
+        case SpawnCategory::Passive: return "passive";
+        case SpawnCategory::Water: return "water";
+        case SpawnCategory::Ambient: return "ambient";
+        case SpawnCategory::Count: break;
+    }
+    return "unknown";
+}
+
+SpawnCategory categoryForSpecies(Species species) {
+    return isHostile(species) ? SpawnCategory::Hostile : SpawnCategory::Passive;
+}
+
+const char* spawnDenialName(SpawnDenial denial) {
+    switch (denial) {
+        case SpawnDenial::None: return "none";
+        case SpawnDenial::CategoryCapReached: return "categoryCapReached";
+        case SpawnDenial::WrongDimension: return "wrongDimension";
+        case SpawnDenial::WrongBiome: return "wrongBiome";
+        case SpawnDenial::WrongTimeOfDay: return "wrongTimeOfDay";
+        case SpawnDenial::HeightOutOfRange: return "heightOutOfRange";
+        case SpawnDenial::UnsuitableGround: return "unsuitableGround";
+        case SpawnDenial::NoHeadroom: return "noHeadroom";
+        case SpawnDenial::BlockLightTooHigh: return "blockLightTooHigh";
+        case SpawnDenial::SkyLightTooHigh: return "skyLightTooHigh";
+        case SpawnDenial::LightTooLow: return "lightTooLow";
+        case SpawnDenial::TooCloseToPlayer: return "tooCloseToPlayer";
+        case SpawnDenial::TooFarFromPlayer: return "tooFarFromPlayer";
+        case SpawnDenial::NoStandableCell: return "noStandableCell";
+        case SpawnDenial::NoRuleMatched: return "noRuleMatched";
+        case SpawnDenial::Count: break;
+    }
+    return "unknown";
+}
+
+bool SpawnRule::allowsBiome(BiomeKind biome) const {
+    if (biomes.empty()) return true;
+    return std::find(biomes.begin(), biomes.end(), biome) != biomes.end();
+}
+
+bool SpawnRule::allowsGround(Block block) const {
+    if (groundBlocks.empty()) return isSolid(block);
+    return std::find(groundBlocks.begin(), groundBlocks.end(), block) != groundBlocks.end();
+}
+
+std::vector<SpawnRule> defaultSpawnRules() {
+    std::vector<SpawnRule> rules;
+    rules.push_back(makeHostileRule(Species::Zombie, 2, 4));
+    rules.push_back(makeHostileRule(Species::Skeleton, 1, 3));
+
+    SpawnRule creeper = makeHostileRule(Species::Creeper, 1, 2);
+    creeper.biomes = {BiomeKind::Plains,   BiomeKind::Forest,   BiomeKind::Taiga,
+                      BiomeKind::Tundra,   BiomeKind::Swamp,    BiomeKind::Jungle,
+                      BiomeKind::Savanna,  BiomeKind::Mountains, BiomeKind::Desert};
+    rules.push_back(creeper);
+
+    rules.push_back(makePassiveRule(
+        Species::Sheep, 2, 4,
+        {BiomeKind::Plains, BiomeKind::Forest, BiomeKind::Savanna, BiomeKind::Taiga}));
+    rules.push_back(makePassiveRule(
+        Species::Cow, 2, 4,
+        {BiomeKind::Plains, BiomeKind::Forest, BiomeKind::Savanna, BiomeKind::Taiga}));
+    rules.push_back(makePassiveRule(
+        Species::Pig, 1, 3, {BiomeKind::Plains, BiomeKind::Forest, BiomeKind::Swamp}));
+    return rules;
+}
+
+const SpawnRule* findSpawnRule(const std::vector<SpawnRule>& rules, Species species) {
+    for (const SpawnRule& rule : rules) {
+        if (rule.species == species) return &rule;
+    }
+    return nullptr;
+}
+