golang-data-structures

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+)

| 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:

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.

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 initia