1 files changed, 154 insertions, 0 deletions
diff --git a/scfg/reader.go b/scfg/reader.go
new file mode 100644
index 0000000..53a0cc4
--- /dev/null
+++ b/scfg/reader.go
@@ -0,0 +1,154 @@
+package scfg
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+// This limits the max block nesting depth to prevent stack overflows.
+const maxNestingDepth = 1000
+
+// Load loads a configuration file.
+func Load(path string) (Block, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	return Read(f)
+}
+
+// Read parses a configuration file from an io.Reader.
+func Read(r io.Reader) (Block, error) {
+	scanner := bufio.NewScanner(r)
+
+	dec := decoder{scanner: scanner}
+	block, closingBrace, err := dec.readBlock()
+	if err != nil {
+		return nil, err
+	} else if closingBrace {
+		return nil, fmt.Errorf("line %v: unexpected '}'", dec.lineno)
+	}
+
+	return block, scanner.Err()
+}
+
+type decoder struct {
+	scanner    *bufio.Scanner
+	lineno     int
+	blockDepth int
+}
+
+// readBlock reads a block. closingBrace is true if parsing stopped on '}'
+// (otherwise, it stopped on Scanner.Scan).
+func (dec *decoder) readBlock() (block Block, closingBrace bool, err error) {
+	dec.blockDepth++
+	defer func() {
+		dec.blockDepth--
+	}()
+
+	if dec.blockDepth >= maxNestingDepth {
+		return nil, false, fmt.Errorf("exceeded max block depth")
+	}
+
+	for dec.scanner.Scan() {
+		dec.lineno++
+
+		l := dec.scanner.Text()
+		words, err := splitWords(l)
+		if err != nil {
+			return nil, false, fmt.Errorf("line %v: %v", dec.lineno, err)
+		} else if len(words) == 0 {
+			continue
+		}
+
+		if len(words) == 1 && l[len(l)-1] == '}' {
+			closingBrace = true
+			break
+		}
+
+		var d *Directive
+		if words[len(words)-1] == "{" && l[len(l)-1] == '{' {
+			words = words[:len(words)-1]
+
+			var name string
+			params := words
+			if len(words) > 0 {
+				name, params = words[0], words[1:]
+			}
+
+			startLineno := dec.lineno
+			childBlock, childClosingBrace, err := dec.readBlock()
+			if err != nil {
+				return nil, false, err
+			} else if !childClosingBrace {
+				return nil, false, fmt.Errorf("line %v: unterminated block", startLineno)
+			}
+
+			// Allows callers to tell apart "no block" and "empty block"
+			if childBlock == nil {
+				childBlock = Block{}
+			}
+
+			d = &Directive{Name: name, Params: params, Children: childBlock, lineno: dec.lineno}
+		} else {
+			d = &Directive{Name: words[0], Params: words[1:], lineno: dec.lineno}
+		}
+		block = append(block, d)
+	}
+
+	return block, closingBrace, nil
+}
+
+func splitWords(l string) ([]string, error) {
+	var (
+		words   []string
+		sb      strings.Builder
+		escape  bool
+		quote   rune
+		wantWSP bool
+	)
+	for _, ch := range l {
+		switch {
+		case escape:
+			sb.WriteRune(ch)
+			escape = false
+		case wantWSP && (ch != ' ' && ch != '\t'):
+			return words, fmt.Errorf("atom not allowed after quoted string")
+		case ch == '\\':
+			escape = true
+		case quote != 0 && ch == quote:
+			quote = 0
+			wantWSP = true
+			if sb.Len() == 0 {
+				words = append(words, "")
+			}
+		case quote == 0 && len(words) == 0 && sb.Len() == 0 && ch == '#':
+			return nil, nil
+		case quote == 0 && (ch == '\'' || ch == '"'):
+			if sb.Len() > 0 {
+				return words, fmt.Errorf("quoted string not allowed after atom")
+			}
+			quote = ch
+		case quote == 0 && (ch == ' ' || ch == '\t'):
+			if sb.Len() > 0 {
+				words = append(words, sb.String())
+			}
+			sb.Reset()
+			wantWSP = false
+		default:
+			sb.WriteRune(ch)
+		}
+	}
+	if quote != 0 {
+		return words, fmt.Errorf("unterminated quoted string")
+	}
+	if sb.Len() > 0 {
+		words = append(words, sb.String())
+	}
+	return words, nil
+}