golang slice扩容原理

本文基于go1.24.0版本

1.基本原理

• 所需容量大于旧容量两倍，扩容至所需容量
• 旧容量小于256，扩容至旧容量的两倍
• 旧容量大于等于256，循环操作扩容旧容量的1.25倍+192直至大于等于所需容量

2.源码

func nextslicecap(newLen, oldCap int) int {
	newcap := oldCap
	doublecap := newcap + newcap
	if newLen > doublecap {
		return newLen
	}

	const threshold = 256
	if oldCap < threshold {
		return doublecap
	}
	for {
		// Transition from growing 2x for small slices
		// to growing 1.25x for large slices. This formula
		// gives a smooth-ish transition between the two.
		newcap += (newcap + 3*threshold) >> 2

		// We need to check `newcap >= newLen` and whether `newcap` overflowed.
		// newLen is guaranteed to be larger than zero, hence
		// when newcap overflows then `uint(newcap) > uint(newLen)`.
		// This allows to check for both with the same comparison.
		if uint(newcap) >= uint(newLen) {
			break
		}
	}

	// Set newcap to the requested cap when
	// the newcap calculation overflowed.
	if newcap <= 0 {
		return newLen
	}
	return newcap
}