Add alpha SSH config parser

The error handling is nonexistent and there's no easy way to get
data out. But we can parse a SSH config file into a Go struct, and
roundtrip that struct back to a file that looks (roughly) the same.

parser.go

@@ -0,0 +1,147 @@
+package ssh_config
+
+import (
+	"fmt"
+	"strings"
+)
+
+type sshParser struct {
+	flow          chan token
+	config        *Config
+	tokensBuffer  []token
+	currentTable  []string
+	seenTableKeys []string
+}
+
+type sshParserStateFn func() sshParserStateFn
+
+// Formats and panics an error message based on a token
+func (p *sshParser) raiseError(tok *token, msg string, args ...interface{}) {
+	panic(tok.Position.String() + ": " + fmt.Sprintf(msg, args...))
+}
+
+func (p *sshParser) run() {
+	for state := p.parseStart; state != nil; {
+		state = state()
+	}
+}
+
+func (p *sshParser) peek() *token {
+	if len(p.tokensBuffer) != 0 {
+		return &(p.tokensBuffer[0])
+	}
+
+	tok, ok := <-p.flow
+	if !ok {
+		return nil
+	}
+	p.tokensBuffer = append(p.tokensBuffer, tok)
+	return &tok
+}
+
+func (p *sshParser) getToken() *token {
+	if len(p.tokensBuffer) != 0 {
+		tok := p.tokensBuffer[0]
+		p.tokensBuffer = p.tokensBuffer[1:]
+		return &tok
+	}
+	tok, ok := <-p.flow
+	if !ok {
+		return nil
+	}
+	return &tok
+}
+
+func (p *sshParser) parseStart() sshParserStateFn {
+	tok := p.peek()
+
+	// end of stream, parsing is finished
+	if tok == nil {
+		return nil
+	}
+
+	switch tok.typ {
+	case tokenComment, tokenEmptyLine:
+		return p.parseComment
+	case tokenKey:
+		return p.parseKV
+	case tokenEOF:
+		return nil
+	default:
+		p.raiseError(tok, fmt.Sprintf("unexpected token %q\n", tok))
+	}
+	return nil
+}
+
+func (p *sshParser) parseKV() sshParserStateFn {
+	key := p.getToken()
+	p.assume(tokenString)
+	val := p.getToken()
+	comment := ""
+	tok := p.peek()
+	if tok.typ == tokenComment && tok.Position.Line == val.Position.Line {
+		tok = p.getToken()
+		comment = tok.val
+	}
+	if key.val == "Host" {
+		patterns := strings.Split(val.val, " ")
+		for i := range patterns {
+			if patterns[i] == "" {
+				patterns = append(patterns[:i], patterns[i+1:]...)
+			}
+		}
+		p.config.Hosts = append(p.config.Hosts, &Host{
+			Patterns:   patterns,
+			Nodes:      make([]Node, 0),
+			EOLComment: comment,
+		})
+		return p.parseStart
+	}
+	lastHost := p.config.Hosts[len(p.config.Hosts)-1]
+	kv := &KV{
+		Key:          key.val,
+		Value:        val.val,
+		Comment:      comment,
+		leadingSpace: uint16(key.Position.Col) - 1,
+		position:     key.Position,
+	}
+	lastHost.Nodes = append(lastHost.Nodes, kv)
+	return p.parseStart
+}
+
+func (p *sshParser) parseComment() sshParserStateFn {
+	comment := p.getToken()
+	lastHost := p.config.Hosts[len(p.config.Hosts)-1]
+	lastHost.Nodes = append(lastHost.Nodes, &Empty{
+		Comment: comment.val,
+		// account for the "#" as well
+		leadingSpace: comment.Position.Col - 2,
+		position:     comment.Position,
+	})
+	return p.parseStart
+}
+
+// assume peeks at the next token and ensures it's the right type
+func (p *sshParser) assume(typ tokenType) {
+	tok := p.peek()
+	if tok == nil {
+		p.raiseError(tok, "was expecting token %s, but token stream is empty", tok)
+	}
+	if tok.typ != typ {
+		p.raiseError(tok, "was expecting token %s, but got %s instead", typ, tok)
+	}
+}
+
+func parseSSH(flow chan token) *Config {
+	result := newConfig()
+	result.position = Position{1, 1}
+	parser := &sshParser{
+		flow:          flow,
+		config:        result,
+		tokensBuffer:  make([]token, 0),
+		currentTable:  make([]string, 0),
+		seenTableKeys: make([]string, 0),
+	}
+	parser.run()
+	return result
+}