version: "1.0.0" 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: "1.1.3" 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

// 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

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

maps Package Quick Reference (Go 1.21+)

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:

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

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.

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

[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

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

References

• Go Data Structures (Russ Cox)
• The Go Memory Model
• Effective Go