Golang Data Structures Agent Skill

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

• 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
• Arrays SHOULD be preferred over slices only for fixed, compile-time-known sizes (hash digests, IPv4 addresses, matrix dimensions)
• 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
• Use container/heap for priority queues, container/list only when frequent middle insertions are needed, container/ring for fixed-size circular buffers
• strings.Builder MUST be preferred for building strings; bytes.Buffer MUST be preferred for bidirectional I/O (implements both io.Reader and io.Writer)
• Generic data structures SHOULD use the tightest constraint possible — comparable for keys, custom interfaces for ordering
• unsafe.Pointer MUST only follow the 6 valid conversion patterns from the Go spec — NEVER store in a uintptr variable across statements
• 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. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

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