--- name: golang-data-structures description: "Golang data structures — slices (internals, capacity growth, preallocation, slices package), maps (internals, hash buckets, maps package), arrays, container/list/heap/ring, strings.Builder vs bytes.Buffer, generic collections, pointers (unsafe.Pointer, weak.Pointer), and copy semantics. Use when choosing or optimizing Go data structures, implementing generic containers, using container/ packages, unsafe or weak pointers, or questioning slice/map internals." user-invocable: true license: MIT compatibility: Designed for Claude Code or similar AI coding agents, and for projects using Golang. metadata: author: samber version: "1.1.1" openclaw: emoji: "🗃️" homepage: https://github.com/samber/cc-skills-golang requires: bins: - go install: [] allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent ---

**Persona:** You are a Go engineer who understands data structure internals. You choose the right structure for the job — not the most familiar one — by reasoning about memory layout, allocation cost, and access patterns.

# Go Data Structures

Built-in and standard library data structures: internals, correct usage, and selection guidance. For safety pitfalls (nil maps, append aliasing, defensive copies) see `samber/cc-skills-golang@golang-safety` skill. For channels and sync primitives see `samber/cc-skills-golang@golang-concurrency` skill. For string/byte/rune choice see `samber/cc-skills-golang@golang-design-patterns` skill.

## Best Practices Summary

1. **Preallocate slices and maps** with `make(T, 0, n)` / `make(map[K]V, n)` when size is known or estimable — avoids repeated growth copies and rehashing 2. **Arrays** SHOULD be preferred over slices only for fixed, compile-time-known sizes (hash digests, IPv4 addresses, matrix dimensions) 3. **NEVER rely on slice capacity growth timing** — the growth algorithm changed between Go versions and may change again; your code should not depend on when a new backing array is allocated 4. **Use `container/heap`** for priority queues, **`container/list`** only when frequent middle insertions are needed, **`container/ring`** for fixed-size circular buffers 5. **`strings.Builder`** MUST be preferred for building strings; **`bytes.Buffer`** MUST be preferred for bidirectional I/O (implements both `io.Reader` and `io.Writer`) 6. Generic data structures SHOULD use the **tightest constraint** possible — `comparable` for keys, custom interfaces for ordering 7. **`unsafe.Pointer`** MUST only follow the 6 valid conversion patterns from the Go spec — NEVER store in a `uintptr` variable across statements 8. **`weak.Pointer[T]`** (Go 1.24+) SHOULD be used for caches and canonicalization maps to allow GC to reclaim entries

## Slice Internals

A slice is a 3-word header: pointer, length, capacity. Multiple slices can share a backing array (→ see `samber/cc-skills-golang@golang-safety` for aliasing traps and the header diagram).

### Capacity Growth

- < 256 elements: capacity doubles - > = 256 elements: grows by ~25% (`newcap += (newcap + 3*256) / 4`) - Each growth copies the entire backing array — O(n)

### Preallocation

```go // Exact size known users := make([]User, 0, len(ids))

// Approximate size known results := make([]Result, 0, estimatedCount)

// Pre-grow before bulk append (Go 1.21+) s = slices.Grow(s, additionalNeeded) ```

### `slices` Package (Go 1.21+)

Key functions: `Sort`/`SortFunc`, `BinarySearch`, `Contains`, `Compact`, `Grow`. For `Clone`, `Equal`, `DeleteFunc` → see `samber/cc-skills-golang@golang-safety` skill.

**[Slice Internals Deep Dive](./references/slice-internals.md)** — Full `slices` package reference, growth mechanics, `len` vs `cap`, header copying, backing array aliasing.

## Map Internals

Maps are hash tables with 8-entry buckets and overflow chains. They are reference types — assigning a map copies the pointer, not the data.

### Preallocation

```go m := make(map[string]*User, len(users)) // avoids rehashing during population ```

### `maps` Package Quick Reference (Go 1.21+)

| Function | Purpose | | ----------------- | ---------------------------- | | `Collect` (1.23+) | Build map from iterator | | `Insert` (1.23+) | Insert entries from iterator | | `All` (1.23+) | Iterator over all entries | | `Keys`, `Values` | Iterators over keys/values |

For `Clone`, `Equal`, sorted iteration → see `samber/cc-skills-golang@golang-safety` skill.

**[Map Internals Deep Dive](./references/map-internals.md)** — How Go maps store and hash data, bucket overflow chains, why maps never shrink (and what to do about it), comparing map performance to alternatives.

## Arrays

Fixed-size, value types. Copied entirely on assignment. Use for compile-time-known sizes:

```go type Digest [32]byte // fixed-size, value type var grid [3][3]int // multi-dimensional cache := map[[2]int]Result{} // arrays are comparable — usable as map keys ```

Prefer slices for everything else — arrays cannot grow and pass by value (expensive for large sizes).

## container/ Standard Library

| Package | Data Structure | Best For | | --- | --- | --- | | `container/list` | Doubly-linked list | LRU caches, frequent middle insertion/removal | | `container/heap` | Min-heap (priority queue) | Top-K, scheduling, Dijkstra | | `container/ring` | Circular buffer | Rolling windows, round-robin | | `bufio` | Buffered reader/writer/scanner | Efficient I/O with small reads/writes |

Container types use `any` (no type safety) — consider generic wrappers. **[Container Patterns, bufio, and Examples](./references/containers.md)** — When to use each container type, generic wrappers to add type safety, and `bufio` patterns for efficient I/O.

## strings.Builder vs bytes.Buffer

Use `strings.Builder` for pure string concatenation (avoids copy on `String()`), `bytes.Buffer` when you need `io.Reader` or byte manipulation. Both support `Grow(n)`. **[Details and comparison](./references/containers.md)**

## Generic Collections (Go 1.18+)

Use the tightest constraint possible. `comparable` for map keys, `cmp.Ordered` for sorting, custom interfaces for domain-specific ordering.

```go type Set[T comparable] map[T]struct{}

func (s Set[T]) Add(v T) { s[v] = struct{}{} } func (s Set[T]) Contains(v T) bool { _, ok := s[v]; return ok } ```

**[Writing Generic Data Structures](./references/generics.md)** — Using Go 1.18+ generics for type-safe containers, understanding constraint satisfaction, and building domain-specific generic types.

## Pointer Types

| Type | Use Case | Zero Value | | --- | --- | --- | | `*T` | Normal indirection, mutation, optional values | `nil` | | `unsafe.Pointer` | FFI, low-level memory layout (6 spec patterns only) | `nil` | | `weak.Pointer[T]` (1.24+) | Caches, canonicalization, weak references | N/A |

**[Pointer Types Deep Dive](./references/pointers.md)** — Normal pointers, `unsafe.Pointer` (the 6 valid spec patterns), and `weak.Pointer[T]` for GC-safe caches that don't prevent cleanup.

## Copy Semantics Quick Reference

| Type | Copy Behavior | Independence | | --- | --- | --- | | `int`, `float`, `bool`, `string` | Value (deep copy) | Fully independent | | `array`, `struct` | Value (deep copy) | Fully independent | | `slice` | Header copied, backing array shared | Use `slices.Clone` | | `map` | Reference copied | Use `maps.Clone` | | `channel` | Reference copied | Same channel | | `*T` (pointer) | Address copied | Same underlying value | | `interface` | Value copied (type + value pair) | Depends on held type |

## Third-Party Libraries

For advanced data structures (trees, sets, queues, stacks) beyond the standard library:

- **`emirpasic/gods`** — comprehensive collection library (trees, sets, lists, stacks, maps, queues) - **`deckarep/golang-set`** — thread-safe and non-thread-safe set implementations - **`gammazero/deque`** — fast double-ended queue

When using third-party libraries, refer to their official documentation and code examples for current API signatures. Context7 can help as a discoverability platform.

## Cross-References

- → See `samber/cc-skills-golang@golang-performance` skill for struct field alignment, memory layout optimization, and cache locality - → See `samber/cc-skills-golang@golang-safety` skill for nil map/slice pitfalls, append aliasing, defensive copying, `slices.Clone`/`Equal` - → See `samber/cc-skills-golang@golang-concurrency` skill for channels, `sync.Map`, `sync.Pool`, and all sync primitives - → See `samber/cc-skills-golang@golang-design-patterns` skill for `string` vs `[]byte` vs `[]rune`, iterators, streaming - → See `samber/cc-skills-golang@golang-structs-interfaces` skill for struct composition, embedding, and generics vs `any` - → See `samber/cc-skills-golang@golang-code-style` skill for slice/map initialization style

## Common Mistakes

| Mistake | Fix | | --- | --- | | Growing a slice in a loop without preallocation | Each growth copies the entire backing array — O(n) per growth. Use `make([]T, 0, n)` or `slices.Grow` | | Using `container/list` when a slice would suffice | Linked lists have poor cache locality (each node is a separate heap allocation). Benchmark first | | `bytes.Buffer` for pure string building | Buffer's `String()` copies the underlying bytes. `strings.Builder` avoids this copy | | `unsafe.Pointer` stored as `uintptr` across statements | GC can move the object between statements — the `uintptr` becomes a dangling reference | | Large struct values in maps (copying overhead) | Map access copies the entire value. Use `map[K]*V` for large value types to avoid the copy |

## References

- [Go Data Structures (Russ Cox)](https://research.swtch.com/godata) - [The Go Memory Model](https://go.dev/ref/mem) - [Effective Go](https://go.dev/doc/effective_go)