Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.swift

let arr = [1, 2, 4, 4, 4, 7, 9]
let target = 4
var lo = 0
var hi = arr.count - 1
var result = -1
while lo <= hi {
	let mid = lo + (hi - lo) / 2
	if arr[mid] == target {
		result = mid
		hi = mid - 1
	} else if arr[mid] < target {
		lo = mid + 1
	} else {
		hi = mid - 1
	}
}
print(result)

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This Swift DSA version keeps the same data and final output as every other DSA book in this wave.