Add Write A Custom Scan Function For File IO as a Go TIL

go/write-a-custom-scan-function-for-file-io.md

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+# Write A Custom Scan Function For File IO
+
+By default a [`bufio.Scanner`](https://pkg.go.dev/bufio#Scanner) will scan
+input line-by-line. In other words, splitting on newlines such that each
+iteration will emit everything up to the next newline character.
+
+We can write our own `SplitFunc` and override the default one by calling
+`scanner.Split` with it. Our custom scan function needs to match the type
+signature of [`SplitFunc`](https://pkg.go.dev/bufio#SplitFunc).
+
+Here is a custom one that emits each individual character but omits the
+newlines.
+
+```go
+func ScanChar(data []byte, atEOF bool) (int, []byte, error) {
+	if atEOF || len(data) == 0 {
+		return 0, nil, nil
+	}
+
+	start := 0
+	for start < len(data) {
+		if !utf8.FullRune(data[start:]) {
+			return 0, nil, nil
+		}
+
+		r, size := utf8.DecodeRune(data[start:])
+		if r == utf8.RuneError {
+			return 0, nil, fmt.Errorf("invalid UTF-8 encoding")
+		}
+
+		if r != '\n' {
+			return start + size, data[start:start+size], nil
+		}
+
+		// found a \n, advance the start position
+		start += size
+	}
+
+	return start, nil, nil
+}
+```
+
+We can then use thi `ScanChar` function with a `bufio.Scanner` like so:
+
+```go
+func ReadFileByCharacter(file io.Reader) {
+	scanner := bufio.NewScanner(file)
+
+	// override default SplitFunc
+	scanner.Split(scanChar)
+
+	for scanner.Scan() {
+		char := scanner.Text()
+
+		fmt.Printf("- %s\n", char)
+	}
+}
+```