From eb7fb2bc95c6333c2415251d3502c488fc5facb1 Mon Sep 17 00:00:00 2001
From: Rose Hogenson <rosehogenson@posteo.net>
Date: Sat, 5 Apr 2025 13:19:31 -0700
Subject: Initial commit

---
 vecdeque.go | 171 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 171 insertions(+)
 create mode 100644 vecdeque.go

(limited to 'vecdeque.go')

diff --git a/vecdeque.go b/vecdeque.go
new file mode 100644
index 0000000..737c99e
--- /dev/null
+++ b/vecdeque.go
@@ -0,0 +1,171 @@
+// Package vecdeque implements a double-ended queue (deque) implemented with a
+// growable ring buffer.
+//
+// This queue has O(1) amortized inserts and removals from both ends of the
+// container. It also has O(1) indexing like a vector.
+package vecdeque
+
+// DQ is a double-ended queue. The zero value is ready for use.
+type DQ[T any] struct {
+	head int
+	buf  []T
+}
+
+// WithCapacity allocates a deque with the given capacity.
+func WithCapacity[T any](cap int) *DQ[T] {
+	return &DQ[T]{buf: make([]T, 0, cap)}
+}
+
+// From creates a new queue using the given slice as the backing buffer.
+func From[S ~[]T, T any](slice S) *DQ[T] {
+	return &DQ[T]{buf: slice}
+}
+
+func (q *DQ[T]) wrapAdd(i, addend int) int {
+	i += addend
+	if i >= cap(q.buf) {
+		return i - cap(q.buf)
+	}
+	return i
+}
+
+func (q *DQ[T]) toPhysicalIdx(i int) int {
+	return q.wrapAdd(q.head, i)
+}
+
+// Get returns the item at position i.
+func (q *DQ[T]) Get(i int) T {
+	return q.buf[:cap(q.buf)][q.toPhysicalIdx(i)]
+}
+
+// Cap returns the number of elements the deque can hold without reallocating.
+func (q *DQ[T]) Cap() int {
+	return cap(q.buf)
+}
+
+// Len returns the number of elements in the deque.
+func (q *DQ[T]) Len() int {
+	return len(q.buf)
+}
+
+// PopFront removes and returns the item at index 0 if the deque is non-empty.
+func (q *DQ[T]) PopFront() (T, bool) {
+	if len(q.buf) == 0 {
+		var zero T
+		return zero, false
+	}
+	oldHead := q.head
+	q.head = q.toPhysicalIdx(1)
+	q.buf = q.buf[:len(q.buf)-1]
+	return q.buf[:cap(q.buf)][oldHead], true
+}
+
+// PopBack removes and returns the last item in the deque if it is non-empty.
+func (q *DQ[T]) PopBack() (T, bool) {
+	if len(q.buf) == 0 {
+		var zero T
+		return zero, false
+	}
+	q.buf = q.buf[:len(q.buf)-1]
+	return q.Get(len(q.buf)), true
+}
+
+// PushFront prepends the given items to the front of the deque.
+func (q *DQ[T]) PushFront(values ...T) {
+	q.Grow(len(values))
+	q.buf = q.buf[:len(q.buf)+len(values)]
+	if q.head >= len(values) {
+		newHead := q.head - len(values)
+		copy(q.buf[newHead:q.head], values)
+		q.head = newHead
+	} else {
+		tailLen := len(values) - q.head
+		copy(q.buf[:q.head], values[tailLen:])
+		copy(q.buf[cap(q.buf)-tailLen:cap(q.buf)], values[:tailLen])
+		q.head = cap(q.buf) - tailLen
+	}
+}
+
+// PushBack appends the given items to the back of the deque.
+func (q *DQ[T]) PushBack(values ...T) {
+	q.Grow(len(values))
+	endIdx := q.wrapAdd(q.head, len(q.buf))
+	if len(values) < cap(q.buf)-endIdx {
+		copy(q.buf[endIdx:endIdx+len(values)], values)
+	} else {
+		headLen := cap(q.buf) - endIdx
+		copy(q.buf[endIdx:cap(q.buf)], values[:headLen])
+		copy(q.buf[:len(values)-headLen], values[headLen:])
+	}
+	q.buf = q.buf[:len(q.buf)+len(values)]
+}
+
+// Grow makes space for at least n more elements to be inserted in the given
+// deque without reallocation.
+func (q *DQ[T]) Grow(n int) {
+	if cap(q.buf)-len(q.buf) >= n {
+		return
+	}
+
+	oldCap := cap(q.buf)
+	q.buf = append(q.buf[:cap(q.buf)], make([]T, n)...)[:len(q.buf)]
+	newCap := cap(q.buf)
+
+	// Move the shortest contiguous section of the ring buffer
+	//
+	// H := head
+	// L := last element (`self.to_physical_idx(self.len - 1)`)
+	//
+	//    H             L
+	//   [o o o o o o o o ]
+	//    H             L
+	// A [o o o o o o o o . . . . . . . . ]
+	//        L H
+	//   [o o o o o o o o ]
+	//          H             L
+	// B [. . . o o o o o o o o . . . . . ]
+	//              L H
+	//   [o o o o o o o o ]
+	//              L                 H
+	// C [o o o o o o . . . . . . . . o o ]
+
+	if q.head <= oldCap-len(q.buf) {
+		// A
+		return
+	}
+	headLen := oldCap - q.head
+	tailLen := len(q.buf) - headLen
+	if headLen > tailLen && newCap-oldCap >= tailLen {
+		// B
+		copy(q.buf[oldCap:oldCap+tailLen], q.buf[:tailLen])
+		return
+	}
+	// C
+	newHead := newCap - headLen
+	copy(q.buf[newHead:newHead+headLen], q.buf[q.head:q.head+headLen])
+	q.head = newHead
+}
+
+// All returns an iterator over the elements in the deque. It does not pop
+// any elements.
+func (q *DQ[T]) All() func(func(int, T) bool) {
+	return func(yield func(int, T) bool) {
+		for i := range q.Len() {
+			if !yield(i, q.Get(i)) {
+				return
+			}
+		}
+	}
+}
+
+// PopAll returns an iterator that consumes all the values in the deque, leaving
+// it empty.
+func (q *DQ[T]) PopAll() func(func(T) bool) {
+	return func(yield func(T) bool) {
+		for val, ok := q.PopFront(); ok; val, ok = q.PopFront() {
+			if !yield(val) {
+				return
+			}
+		}
+	}
+}
-- 
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