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json lib for lua and C
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README.md

lua-resty-json

Json lib for lua and C. The C interface is depicted by ljson_parser.h; while the Lua interface is implemented by json_decoder.lua. The lua interface is built on top of C implementation, and it's implemented using FFI instead of Lua C-API.

Following is an example of Lua usage:

local ljson_decoder = require 'json_decoder'
local instance = ljson_decoder.create()
local result, err = ljson_decoder.parse(instance, line)

Performance

As of I write this README.md, I compare this work against cjson using few real-world json strings. For string-array intensive jsons, our decoder is normally 30% - 50% ahead of cjson. While for the hash-table intensive input, we are only 10-30% better. In an extreme example where there is a super long string, we see 5X speedup. The performance is measured with luajit 2.1.

So far we pay lots of attention to string handling, and did not get chance to improve following aspects:

  • Parse floating point number quickly. so far we rely on strtod() to do the dirty job. Unfortunately, the strtod() seems to be pretty slow.

  • Efficiently skip white-spaces between tokens.

  • More efficient memory allocation. We are currently using mempool which allocate a big chunk and the subsequent memory allocation requests are served by carving block out of the chunk. It works pretty well for small to medium-sized JSON input (say under 100k in size); however, the memory allocation overhead is still high (primarily due to the cost of allocating big chunks) for big JSONs.

Floating Point Number

The way we handle following situations may not be what you expect, but the JSON SPEC does not seem to articulate how to handle these situations right either.

  • literal -0 is interpreted as integer 0, instead of floating point 0.0.

  • -0.0 is interperted as floating point -0.0.

  • If a literal is beyond the range of double-precision, we consider it as overflow/underflow; we do not try to represent the literal using long double or quadruple.

  • We rely on strtod() to parse literal (in strict mode), which, I guess, is using default rounding mode or the mode designated by the appliction which call the decoder.

  • We try to represent literals in signed 64-bit interger whenever possible. But the numbers like 1E6 is still represented as floating point as we currently rely on strtod() for handling scientific notation.

TODO

  • Continue to improve floating point parsing.
  • Improve testing, and add more testing.
  • Improve hashtab parsing performance (I almost have not yet got chance tune its performance when I write this comment).
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