1 files changed, 50 insertions, 0 deletions
diff --git a/shortcodes.go b/shortcodes.go
new file mode 100644
index 0000000..978b39b
--- /dev/null
+++ b/shortcodes.go
@@ -0,0 +1,50 @@
+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)
+}