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Parse null,true/false directly from the buffer, avoid copying #25492

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Parse null,true/false directly from the buffer, avoid copying #25492

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41 changes: 28 additions & 13 deletions lib/pure/parsejson.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -343,13 +343,33 @@ proc parseNumber(my: var JsonParser) =
inc(pos)
my.bufpos = pos

proc parseName(my: var JsonParser) =
var pos = my.bufpos
if my.buf[pos] in IdentStartChars:
while my.buf[pos] in IdentChars:
add(my.a, my.buf[pos])
inc(pos)
my.bufpos = pos
proc parseKeyword(my: var JsonParser): TokKind =
let pos = my.bufpos
case my.buf[pos]
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@arnetheduck arnetheduck Feb 9, 2026 •
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what happens when my.buf.len <= pos + len(match)?

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@planetis-m planetis-m Feb 9, 2026

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Good question, I suppose it reads the terminating character and works correctly.

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@arnetheduck arnetheduck Feb 9, 2026

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... or, more likely, [] raises a defect?

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@planetis-m planetis-m Feb 9, 2026

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I think the way this parser is built and also looking at lexbase.nim, it makes the assumption that [] doesnt perform range checks.

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@arnetheduck arnetheduck Feb 9, 2026

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ah, I see 👍 there's an extra non-ident character injected always, it seems so although [] performs a range check it should not be violated.

of 'n':
if my.buf[pos + 1] == 'u' and my.buf[pos + 2] == 'l' and
my.buf[pos + 3] == 'l' and my.buf[pos + 4] notin IdentChars:
my.bufpos = pos + 4
return tkNull
of 't':
if my.buf[pos + 1] == 'r' and my.buf[pos + 2] == 'u' and
my.buf[pos + 3] == 'e' and my.buf[pos + 4] notin IdentChars:
my.bufpos = pos + 4
return tkTrue
of 'f':
if my.buf[pos + 1] == 'a' and my.buf[pos + 2] == 'l' and
my.buf[pos + 3] == 's' and my.buf[pos + 4] == 'e' and
my.buf[pos + 5] notin IdentChars:
my.bufpos = pos + 5
return tkFalse
else:
discard

var endPos = pos
while my.buf[endPos] in IdentChars:
inc(endPos)
my.bufpos = endPos
result = tkError

proc getTok*(my: var JsonParser): TokKind =
setLen(my.a, 0)
Expand Down Expand Up @@ -384,12 +404,7 @@ proc getTok*(my: var JsonParser): TokKind =
of '\0':
result = tkEof
of 'a'..'z', 'A'..'Z', '_':
parseName(my)
case my.a
of "null": result = tkNull
of "true": result = tkTrue
of "false": result = tkFalse
else: result = tkError
result = parseKeyword(my)
else:
inc(my.bufpos)
result = tkError
Expand Down
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