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.teamai/skills/common/golang-data-structures/references/map-internals.md

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+# Map Internals Deep Dive
+
+## Hash Table Structure
+
+Go maps use hash tables with bucket-based collision resolution. The map header holds:
+
+- `count` — number of entries
+- `B` — log₂ of bucket count (2^B buckets total)
+- `buckets` — pointer to bucket array
+- `oldbuckets` — pointer to old buckets during growth
+
+Each bucket holds 8 key-value pairs. Keys and values are stored in separate arrays within buckets to minimize padding waste.
+
+## Memory Growth and Capacity
+
+- **Load factor threshold**: 6.5 entries per bucket triggers growth (sweet spot between memory efficiency and collision performance)
+- **Overflow bucket chains** also trigger growth if too long (prevents O(1)→O(n) degradation)
+- **Bucket count doubles**: 2^B → 2^(B+1) (efficient rehashing with powers of 2)
+- **Incremental evacuation**: Old and new buckets coexist during growth; entries move lazily during operations to avoid GC pauses
+- **No `cap()` function**: Capacity depends on hash distribution and load factor, not a fixed limit. Preallocation (`make(map[string]int, expectedSize)`) is worthwhile for large maps to avoid repeated growth cycles
+
+## Preallocation
+
+```go
+// Without preallocation — multiple growths as entries are added
+m := map[string]int{}
+
+// With preallocation — allocates enough buckets upfront
+m := make(map[string]int, expectedSize)
+```
+
+Preallocation avoids repeated growths. The hint is approximate — Go allocates 2^B buckets where 2^B \* 6.5 >= hint.
+
+## Pointers vs Values
+
+For large value types, storing pointers reduces copy overhead:
+
+```go
+// Large struct — copied on every read/write
+m := map[string]BigStruct{}  // copies large struct
+
+// Pointer — only pointer is copied
+m := map[string]*BigStruct{} // copies 8-byte pointer
+```
+
+Trade-off: pointer maps add GC pressure. For small structs (< 128 bytes), value maps are typically faster.
+
+## `maps` Package (Go 1.21+)
+
+| Function | Description |
+| --- | --- |
+| `Clone`, `Equal`, `EqualFunc` | Shallow copy and equality comparison |
+| `Keys`, `Values`, `All` (1.23+) | Iterators over keys, values, or pairs |
+| `Collect`, `Insert` (1.23+) | Build maps from iterators or insert entries |
+
+See `samber/cc-skills-golang@golang-safety` skill for `Clone`, `Equal`, and sorted iteration patterns.
+
+## Map Key Requirements
+
+Map keys must be comparable (`==` must work). This includes:
+
+- All numeric types, `string`, `bool`
+- Pointers, channels, interfaces (compared by identity)
+- Arrays of comparable types
+- Structs where all fields are comparable
+
+Slices, maps, and functions **cannot** be map keys.