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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2021-2022 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//
extension PEG {
struct Grammar {
var environment: Dictionary<String, Pattern>
var start: Pattern
}
struct Interpreter<C: Collection> where C.Element == Element {
var grammar: Grammar
func consume(_ c: C, from: C.Index) -> C.Index? {
fatalError()
}
func consume(_ c: C) -> C.Index? {
consume(c, from: c.startIndex)
}
func find(_ c: C) -> Range<C.Index>? {
// TODO: more efficient to have native support
var idx = c.startIndex
while idx != c.endIndex {
if let endIdx = consume(c, from: idx) {
return idx ..< endIdx
}
c.formIndex(after: &idx)
}
return nil
}
}
}
extension PEG.Program {
func consume<C: Collection>(
_ c: C
) -> C.Index? where C.Element == Element {
_consume(c, from: c.startIndex, environment[start]!, environment)
}
}
func _consume<C: Collection>(
_ c: C,
from idx: C.Index,
_ pattern: PEG<C.Element>.Pattern,
_ environment: PEG<C.Element>.Environment
) -> C.Index? {
var captures = Array<Range<C.Index>>()
return _consume(c, from: idx, pattern, environment, &captures)
}
func _consume<C: Collection>(
_ c: C,
from idx: C.Index,
_ pattern: PEG<C.Element>.Pattern,
_ environment: PEG<C.Element>.Environment,
_ captures: inout Array<Range<C.Index>>
) -> C.Index? {
typealias Pattern = PEG<C.Element>.Pattern
var idx = idx
func consume(_ p: Pattern, from idx: C.Index) -> C.Index? {
_consume(c, from: idx, p, environment, &captures)
}
func consumeIf(_ p: (C.Element) -> Bool) -> C.Index? {
guard idx < c.endIndex, p(c[idx]) else { return nil }
return c.index(after: idx)
}
func consumePrefix<Prefix: Sequence>(
_ p: Prefix
) -> C.Index? where Prefix.Element == C.Element {
var idx = idx
for e in p {
guard idx < c.endIndex, e == c[idx] else { return nil }
c.formIndex(after: &idx)
}
return idx
}
func consumeMany(_ p: Pattern, atLeast: Int) -> C.Index? {
var counter = 0
var idx = idx
while let nextIdx = consume(p, from: idx) {
counter += 1
idx = nextIdx
}
guard counter >= atLeast else { return nil }
return idx
}
func peek(_ p: Pattern) -> Bool {
nil != consume(p, from: idx)
}
switch pattern {
// Terminals
case .success: return idx
case .failure: return nil
case .end: return idx == c.endIndex ? idx : nil
// Consume a single unit of input
case .any: return consumeIf { _ in true }
case .element(let e): return consumeIf { e == $0 }
case .charactetSet(let p): return consumeIf(p)
// Consume many inputs
case .literal(let lit): return consumePrefix(lit)
case .repeat(let e, atLeast: let atLeast):
return consumeMany(e, atLeast: atLeast)
case .repeatRange(_, atLeast: _, atMost: _):
fatalError()
// Assertions (does not consume)
case .and(let p):
return peek(p) ? idx : nil
case .not(let p):
return peek(p) ? nil : idx
// Combinations of patterns
case .orderedChoice(let p1, let p2):
return consume(p1, from: idx) ?? consume(p2, from: idx)
case .concat(let ps):
for p in ps {
guard let resume = consume(p, from: idx) else {
return nil
}
idx = resume
}
return idx
case .difference(let p1, let p2):
guard !peek(p2) else { return nil }
return consume(p1, from: idx)
case .variable(let name):
return consume(environment[name]!, from: idx)
case .capture(let p):
// TODO: Are captured pre-order or post-order?
guard let endIdx = consume(p, from: idx) else { return nil }
captures.append(idx ..< endIdx)
return endIdx
}
}
// Match some subsequence within the collection
func _find<C: Collection>(
_ c: C,
from idx: C.Index,
_ pattern: PEG<C.Element>.Pattern,
_ environment: PEG<C.Element>.Environment,
_ captures: inout Array<Range<C.Index>>
) -> Range<C.Index>? {
// TODO: more efficient to have native support
var idx = c.startIndex
while idx != c.endIndex {
if let endIdx = _consume(c, from: idx, pattern, environment, &captures) {
return idx ..< endIdx
}
c.formIndex(after: &idx)
}
return nil
}
// Convenience: gather up the result of repeated consumption
func _gather<C: Collection>(
_ c: C,
_ pattern: PEG<C.Element>.Pattern,
_ environment: PEG<C.Element>.Environment
) -> [C.SubSequence] {
var idx = c.startIndex
var result = Array<C.SubSequence>()
while let end = _consume(c, from: idx, pattern, environment) {
result.append(c[idx ..< end])
idx = end
}
return result
}
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