package main

import (
	"encoding/csv"
	"fmt"
	"io"
	"os"
)

// getURLFromShortcode looks for the shortcode inside the file
// specified by filename (default "urls.csv" in the top-level project
// directory), and returns the associated URL.
//
// Right now we perform an O(n) search across the entire file just to
// get a single URL, so there's room for improvement here.
func getURLFromShortcode(filename, shortcode string) (string, error) {
	f, err := os.Open(filename)
	if err != nil {
		return "", fmt.Errorf("can't open shortcode file: %w", err)
	}
	defer f.Close()

	r := csv.NewReader(f)

	// Read, vet, and discard the column-header row.
	header, err := r.Read()
	if err != nil {
		return "", fmt.Errorf("missing header line: %w", err)
	}

	if header[0] != "shortcode" || header[1] != "url" {
		return "", fmt.Errorf("invalid CSV header: %v", header)
	}

	for i := 1; ; i++ {
		record, err := r.Read()
		if err == io.EOF {
			break
		}
		if err != nil {
			return "", fmt.Errorf("error reading %s, row %d: %w", filename, i, err)
		}

		if record[0] == shortcode {
			return record[1], nil
		}
	}

	return "", fmt.Errorf("no URL with shortcode %s", shortcode)
}