paperspec/tools/oracle/jcs.py

"""Canonical JSON, RFC 8785 (JSON Canonicalization Scheme).

Non-normative.
The normative rule is spec/01-conventions.md.
Most values serialize through the standard library's C encoder, which escapes strings exactly as RFC 8785 does.
Values it would write differently take the exact path below: floats, integers beyond 2^53, and member names
outside the Basic Multilingual Plane, which RFC 8785 sorts by UTF-16 code units rather than by code points.
"""

import json
import math

_C = json.JSONEncoder(ensure_ascii=False, separators=(",", ":"), sort_keys=True, allow_nan=False)
_EXACT_INT = 2**53

_ESC = {
    0x08: "\\b",
    0x09: "\\t",
    0x0A: "\\n",
    0x0C: "\\f",
    0x0D: "\\r",
    0x22: '\\"',
    0x5C: "\\\\",
}


def _string(s):
    out = ['"']
    for ch in s:
        cp = ord(ch)
        if 0xD800 <= cp <= 0xDFFF:
            raise ValueError("lone surrogate in string")
        if cp in _ESC:
            out.append(_ESC[cp])
        elif cp < 0x20:
            out.append("\\u%04x" % cp)
        else:
            out.append(ch)
    out.append('"')
    return "".join(out)


def _number(x):
    """ECMAScript Number.prototype.toString, as RFC 8785 requires."""
    if isinstance(x, bool):
        raise TypeError("bool is not a number")
    if isinstance(x, int):
        if abs(x) > 2**53:
            raise ValueError("integer outside the IEEE 754 exact range")
        return str(x)
    if not math.isfinite(x):
        raise ValueError("NaN and Infinity are not JSON")
    if x == 0:
        return "0"
    if x < 0:
        return "-" + _number(-x)
    # Shortest round-trip digits, as ECMAScript requires.
    r = repr(x)
    if "e" in r or "E" in r:
        mant, exp = r.lower().split("e")
        exp = int(exp)
    else:
        mant, exp = r, 0
    if "." in mant:
        ip, fp = mant.split(".")
    else:
        ip, fp = mant, ""
    digits = (ip + fp).lstrip("0")
    # Position of the decimal point relative to the start of `digits`.
    lead_zeros = len(ip + fp) - len((ip + fp).lstrip("0"))
    n = len(ip) - lead_zeros + exp
    digits = digits.rstrip("0") or "0"
    k = len(digits)
    if k <= n <= 21:
        return digits + "0" * (n - k)
    if 0 < n <= 21:
        return digits[:n] + "." + digits[n:]
    if -6 < n <= 0:
        return "0." + "0" * (-n) + digits
    e = n - 1
    sign = "+" if e >= 0 else "-"
    if k == 1:
        return digits + "e" + sign + str(abs(e))
    return digits[0] + "." + digits[1:] + "e" + sign + str(abs(e))


def _key(s):
    # RFC 8785 sorts member names by their UTF-16 code units.
    return s.encode("utf-16-be")


def _fast(v):
    """True when the C encoder writes `v` exactly as RFC 8785 does."""
    t = type(v)
    if t is str or v is None or t is bool:
        return True
    if t is int:
        return -_EXACT_INT <= v <= _EXACT_INT
    if t is dict:
        for k, x in v.items():
            if type(k) is not str or (not k.isascii() and max(k) > "\uffff") or not _fast(x):
                return False
        return True
    if t is list or t is tuple:
        return all(_fast(x) for x in v)
    return False


def dumps(value):
    """Serialize `value` to canonical JSON text."""
    if _fast(value):
        text = _C.encode(value)
        if not text.isascii():
            try:
                text.encode("utf-8")
            except UnicodeEncodeError:
                raise ValueError("lone surrogate in string") from None
        return text
    return _exact(value)


def _exact(value):
    if value is None:
        return "null"
    if value is True:
        return "true"
    if value is False:
        return "false"
    if isinstance(value, str):
        return _string(value)
    if isinstance(value, (int, float)):
        return _number(value)
    if isinstance(value, (list, tuple)):
        return "[" + ",".join(dumps(v) for v in value) + "]"
    if isinstance(value, dict):
        for k in value:
            if not isinstance(k, str):
                raise TypeError("object keys must be strings")
        items = sorted(value.items(), key=lambda kv: _key(kv[0]))
        return "{" + ",".join(_string(k) + ":" + dumps(v) for k, v in items) + "}"
    raise TypeError("not JSON: %r" % type(value))


def dumpb(value):
    """Canonical JSON as UTF-8 bytes."""
    return dumps(value).encode("utf-8")