From b3f4186f2b8e992f56f898784b1cd28bfd7550ca Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@lukeshu.com>
Date: Sun, 29 Jan 2023 20:59:37 -0700
Subject: Invent "barriers" instead of nesting parsers

---
 internal/parse.go | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 117 insertions(+), 3 deletions(-)

(limited to 'internal/parse.go')

diff --git a/internal/parse.go b/internal/parse.go
index 9db57fb..36db4a9 100644
--- a/internal/parse.go
+++ b/internal/parse.go
@@ -313,6 +313,13 @@ type Parser struct {
 	//   a       ["x","y"
 	//           ["x","y"]
 	stack []RuneType
+
+	barriers []barrier
+}
+
+type barrier struct {
+	closed bool
+	stack  []RuneType
 }
 
 func (par *Parser) init() {
@@ -345,8 +352,22 @@ func (par *Parser) stackString() string {
 	return buf.String()
 }
 
+func (par *Parser) depth() int {
+	n := len(par.stack)
+	for _, barrier := range par.barriers {
+		n += len(barrier.stack)
+	}
+	return n
+}
+
 func (par *Parser) StackIsEmpty() bool {
-	return len(par.stack) == 0 || (len(par.stack) == 1 && par.stack[0] == runeTypeAny)
+	if len(par.barriers) > 0 {
+		return false
+	}
+	if len(par.stack) == 0 {
+		return true
+	}
+	return len(par.stack) == 1 && par.stack[0] == runeTypeAny
 }
 
 func (par *Parser) StackSize() int {
@@ -360,6 +381,99 @@ func (par *Parser) Reset() {
 	}
 }
 
+// PushReadBarrier causes the parser to expect EOF once the end of the
+// element that is started by the current top-of-stack is reached,
+// until this is un-done with PopBarrier.  It essentially turns the
+// parser in to a sub-parser.
+//
+// PushReadBarrier may only be called at the beginning of an element,
+// whether that be
+//
+//   - runeTypeAny
+//   - RuneTypeObjectBeg
+//   - RuneTypeArrayBeg
+//   - RuneTypeStringBeg
+//   - RuneTypeNumberIntNeg, RuneTypeNumberIntZero, RuneTypeNumberIntDig
+//   - RuneTypeTrueT
+//   - RuneTypeFalseF
+//   - RuneTypeNullN
+func (par *Parser) PushReadBarrier() {
+	// Sanity checking.
+	par.init()
+	if len(par.stack) == 0 {
+		panic(errors.New("illegal PushReadBarrier call: empty stack"))
+	}
+	curState := par.stack[len(par.stack)-1]
+	switch curState {
+	case runeTypeAny,
+		RuneTypeObjectBeg,
+		RuneTypeArrayBeg,
+		RuneTypeStringBeg,
+		RuneTypeNumberIntNeg, RuneTypeNumberIntZero, RuneTypeNumberIntDig,
+		RuneTypeTrueT,
+		RuneTypeFalseF,
+		RuneTypeNullN:
+		// OK
+	default:
+		panic(fmt.Errorf("illegal PushReadBarrier call: %q", curState))
+	}
+	// Actually push.
+	par.barriers = append(par.barriers, barrier{
+		closed: par.closed,
+		stack:  par.stack[:len(par.stack)-1],
+	})
+	par.stack = []RuneType{curState}
+}
+
+// PushWriteBarrier causes the parser to expect EOF once the end of
+// the about-to-start element is reached, until this is un-done with
+// PopBarrier.  It essentially turns the parser in to a sub-parser.
+//
+// PushWriteBarrier may only be called at the places where an element
+// of any type may start:
+//
+//   - runeTypeAny for top-level and object-value elements
+//   - RuneTypeArrayBeg for array-item elements
+//
+// PushWriteBarrier signals intent to write an element; if it is
+// called in a place where an element is optional (at the beginning of
+// an array), it becomes a syntax error to not write the element.
+func (par *Parser) PushWriteBarrier() {
+	par.init()
+	if len(par.stack) == 0 {
+		panic(errors.New("illegal PushWriteBarrier call: empty stack"))
+	}
+	switch par.stack[len(par.stack)-1] {
+	case runeTypeAny:
+		par.popState()
+		par.barriers = append(par.barriers, barrier{
+			closed: par.closed,
+			stack:  par.stack,
+		})
+		par.stack = []RuneType{runeTypeAny}
+	case RuneTypeArrayBeg:
+		par.replaceState(RuneTypeArrayComma)
+		par.barriers = append(par.barriers, barrier{
+			closed: par.closed,
+			stack:  par.stack,
+		})
+		par.stack = []RuneType{runeTypeAny}
+	default:
+		panic(fmt.Errorf("illegal PushWriteBarrier call: %q", par.stack[len(par.stack)-1]))
+	}
+}
+
+// PopBarrier reverses a call to PushReadBarrier or PushWriteBarrier.
+func (par *Parser) PopBarrier() {
+	if len(par.barriers) == 0 {
+		panic(errors.New("illegal PopBarrier call: empty barrier stack"))
+	}
+	barrier := par.barriers[len(par.barriers)-1]
+	par.barriers = par.barriers[:len(par.barriers)-1]
+	par.closed = barrier.closed
+	par.stack = append(barrier.stack, par.stack...)
+}
+
 // HandleEOF feeds EOF to the Parser.  The returned RuneType is either
 // RuneTypeEOF or RuneTypeError.
 //
@@ -439,12 +553,12 @@ func (par *Parser) HandleRune(c rune) (RuneType, error) {
 		case 0x0020, 0x000A, 0x000D, 0x0009:
 			return RuneTypeSpace, nil
 		case '{':
-			if par.MaxDepth > 0 && len(par.stack) > par.MaxDepth {
+			if par.MaxDepth > 0 && par.depth() > par.MaxDepth {
 				return RuneTypeError, ErrParserExceededMaxDepth
 			}
 			return par.replaceState(RuneTypeObjectBeg), nil
 		case '[':
-			if par.MaxDepth > 0 && len(par.stack) > par.MaxDepth {
+			if par.MaxDepth > 0 && par.depth() > par.MaxDepth {
 				return RuneTypeError, ErrParserExceededMaxDepth
 			}
 			return par.replaceState(RuneTypeArrayBeg), nil
-- 
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