Implement a concurrent word count across multiple files — a classic Go interview puzzle.

This exercise tests goroutine spawning, WaitGroup usage, channel fan-in, and safe result aggregation. It is a common live-coding assignment in Go technical screens.

package main

import (
    "bufio"
    "context"
    "fmt"
    "os"
    "sync"
)

type FileCount struct {
    File  string
    Words int
    Err   error
}

func countWords(ctx context.Context, path string) FileCount {
    f, err := os.Open(path)
    if err != nil { return FileCount{File: path, Err: err} }
    defer f.Close()

    scanner := bufio.NewScanner(f)
    scanner.Split(bufio.ScanWords)
    count := 0
    for scanner.Scan() {
        select {
        case <-ctx.Done(): return FileCount{File: path, Err: ctx.Err()}
        default:
        }
        count++
    }
    return FileCount{File: path, Words: count, Err: scanner.Err()}
}

func parallelWordCount(ctx context.Context, files []string) (int, []error) {
    results := make(chan FileCount, len(files)) // buffered — no goroutine blocks

    var wg sync.WaitGroup
    for _, f := range files {
        wg.Add(1)
        go func(path string) {
            defer wg.Done()
            results <- countWords(ctx, path)
        }(f)
    }

    go func() { wg.Wait(); close(results) }()

    total := 0
    var errs []error
    for r := range results {
        if r.Err != nil {
            errs = append(errs, fmt.Errorf("%s: %w", r.File, r.Err))
            continue
        }
        total += r.Words
    }
    return total, errs
}

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