teamai-test/.teamai/skills/common/golang-data-structures/references/slice-internals.md

Slice Internals

Memory Layout

A slice is a 24-byte header (3 machine words):

• Pointer — points to backing array (heap-allocated)
• Length — number of elements in use
• Capacity — allocated size of backing array

Assigning or passing a slice copies the 24-byte header, not the backing array. Both the original and copy point to the same underlying data—mutations are visible to both.

Capacity Growth

When append exceeds capacity:

• oldCap < 256: double capacity
• oldCap ≥ 256: grow ~25% (oldCap + (oldCap + 3*256) / 4)

Growth Cost

Each growth is O(n) — the entire array is copied to a new location. For a slice growing from 0 to N elements one at a time, the amortized cost per append is O(1), but the total copies are roughly 2N. Preallocation eliminates all intermediate copies:

// Known size — direct indexing
out := make([]Result, len(input))
for i, v := range input {
    out[i] = transform(v)
}

// Approximate size
out := make([]Result, 0, len(input)*2)
for _, v := range input {
    out = append(out, transform(v))
}

slices Package (Go 1.21+)

Category	Key Functions
Sort	Sort, SortFunc, SortStableFunc, IsSorted
Search	BinarySearch, BinarySearchFunc, Contains, Index, IndexFunc
Mutate	Insert, Delete, Replace, Compact, Reverse, Grow, Clip
Create	Concat (1.22+), Repeat (1.23+), Chunk (1.23+)
Compare	Clone, Equal, EqualFunc, Compare, DeleteFunc

copy() vs append() vs slices.Clone()

Operation	Use When
copy(dst, src)	Copying into pre-allocated slice
append(dst, src...)	Appending to a slice
slices.Clone(s)	Creating independent copy
s[:len(s):len(s)]	Preventing append aliasing