combo

README

combo

Combinatorial constructions: Cartesian products, permutations, combinations, and power sets.

All functions eagerly allocate the full result in memory. CartesianProduct returns pair.Pair values; use CartesianProductWith to produce your own result type directly and avoid intermediate pairs. Results are slice.Mapper — chain directly with .KeepIf, .Convert, etc.

// Before: nested loop to generate all size/color combinations
var pairs []Option
for _, size := range sizes {
    for _, color := range colors {
        pairs = append(pairs, Option{size, color})
    }
}

// After: one call — produces domain objects directly
pairs := combo.CartesianProductWith(sizes, colors, NewOption)

What It Looks Like

// All pairs from two slices
pairs := combo.CartesianProduct([]int{1, 2}, []string{"a", "b"})
// [{1 a} {1 b} {2 a} {2 b}]

// All orderings (n! results)
combo.Permutations([]int{1, 2, 3})
// [[1 2 3] [1 3 2] [2 1 3] [2 3 1] [3 1 2] [3 2 1]]

// k-element subsets — C(n,k) results
combo.Combinations([]string{"a", "b", "c", "d"}, 2)
// [[a b] [a c] [a d] [b c] [b d] [c d]]

// All subsets (2^n results)
combo.PowerSet([]int{1, 2})
// [[] [2] [1] [1 2]]

// Fluent chain — results are slice.Mapper, so chain directly
// Keep only 2-element combinations that sum to at least 5
combo.Combinations([]int{1, 2, 3, 4}, 2).KeepIf(sumsToAtLeast5)
// [[1 4] [2 3] [2 4] [3 4]]

Growth Rates

All results are fully materialized in memory. Compute the result size before calling:

Function	Results	n=5	n=10	n=15
Permutations	n!	120	3.6M	1.3T
PowerSet	2^n	32	1,024	32,768
Combinations(n, k)	C(n, k)	C(5,2)=10	C(10,5)=252	C(15,7)=6,435
CartesianProduct	a * b	depends on inputs

Use only for small inputs unless you've computed the result size and memory cost. Permutations becomes impractical above ~10-12 elements. PowerSet grows quickly. Combinations can also be large near midpoint k.

Empty and Invalid Input

Function	Empty/nil input	Invalid args
CartesianProduct	nil if either input is empty/nil	—
CartesianProductWith	nil if either input is empty/nil	fn must not be nil
Permutations	[[]] (one empty permutation)	—
Combinations	[[]] for k=0	nil if k < 0 or k > len(items)
PowerSet	[[]] (one empty subset)	—

Operations

• CartesianProduct[A, B any]([]A, []B) slice.Mapper[pair.Pair[A, B]] — all pairs
• CartesianProductWith[A, B, R any]([]A, []B, func(A, B) R) slice.Mapper[R] — all pairs, transformed (avoids intermediate pair.Pair allocation)
• Permutations[T any]([]T) slice.Mapper[[]T] — all orderings
• Combinations[T any]([]T, int) slice.Mapper[[]T] — k-element subsets, preserving order
• PowerSet[T any]([]T) slice.Mapper[[]T] — all subsets

See pkg.go.dev for complete API documentation and the main README for installation.

Documentation

Overview

Package combo provides combinatorial constructions: Cartesian products, permutations, combinations, and power sets.

Index

• func CartesianProduct[A, B any](a []A, b []B) slice.Mapper[pair.Pair[A, B]]
• func CartesianProductWith[A, B, R any](a []A, b []B, fn func(A, B) R) slice.Mapper[R]
• func Combinations[T any](items []T, k int) slice.Mapper[[]T]
• func Permutations[T any](items []T) slice.Mapper[[]T]
• func PowerSet[T any](items []T) slice.Mapper[[]T]

Functions

func CartesianProduct

func CartesianProduct[A, B any](a []A, b []B) slice.Mapper[pair.Pair[A, B]]

CartesianProduct returns all pairs from a and b.

func CartesianProductWith

func CartesianProductWith[A, B, R any](a []A, b []B, fn func(A, B) R) slice.Mapper[R]

CartesianProductWith applies fn to every (a, b) pair. Avoids intermediate Pair allocation when the caller transforms immediately. Panics if fn is nil.

func Combinations

func Combinations[T any](items []T, k int) slice.Mapper[[]T]

Combinations returns all k-element subsets of items, preserving order. Returns C(n,k) results.

func Permutations

func Permutations[T any](items []T) slice.Mapper[[]T]

Permutations returns all orderings of items. Returns n! results — use only for small inputs.

func PowerSet

func PowerSet[T any](items []T) slice.Mapper[[]T]

PowerSet returns all subsets of items. Returns 2^n results — use only for small inputs.