golang-data-structures

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	false
license	MIT
compatibility	Designed for Claude Code or similar AI coding agents, and for projects using Golang.
metadata	{"author":"samber","version":"1.1.1"}
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 — 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+)

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 — 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

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 — 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

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 — 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 — 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)
• The Go Memory Model
• Effective Go