avoid ostream sentries per json string character

we don't have to create an ostream sentry object for every character of a JSON
string we write. format a bunch of characters and flush them to the stream all
at once instead.

this doesn't affect small numbers of string characters, but larger numbers of
total JSON string characters written gain a lot. at 1MB of total string written
we gain almost 30%, at 16MB it's almost a factor of 3x. large numbers of JSON
string characters do occur naturally in a nixos system evaluation to generate
documentation (though this is now somewhat mitigated by caching the largest part
of nixos option docs).

benchmarked with

  hyperfine 'nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) {e})"' --warmup 1 -L e 1,4,256,4096,65536

before:

  Benchmark 1: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 1)"
    Time (mean ± σ):      12.5 ms ±   0.2 ms    [User: 9.2 ms, System: 4.0 ms]
    Range (min … max):    11.9 ms …  13.1 ms    223 runs

  Benchmark 2: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4)"
    Time (mean ± σ):      12.5 ms ±   0.2 ms    [User: 9.3 ms, System: 3.8 ms]
    Range (min … max):    11.9 ms …  13.2 ms    220 runs

  Benchmark 3: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 256)"
    Time (mean ± σ):      13.2 ms ±   0.3 ms    [User: 9.8 ms, System: 4.0 ms]
    Range (min … max):    12.6 ms …  14.3 ms    205 runs

  Benchmark 4: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4096)"
    Time (mean ± σ):      24.0 ms ±   0.4 ms    [User: 19.4 ms, System: 5.2 ms]
    Range (min … max):    22.7 ms …  25.8 ms    119 runs

  Benchmark 5: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 65536)"
    Time (mean ± σ):     196.0 ms ±   3.7 ms    [User: 171.2 ms, System: 25.8 ms]
    Range (min … max):   190.6 ms … 201.5 ms    14 runs

after:

  Benchmark 1: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 1)"
    Time (mean ± σ):      12.4 ms ±   0.3 ms    [User: 9.1 ms, System: 4.0 ms]
    Range (min … max):    11.7 ms …  13.3 ms    204 runs

  Benchmark 2: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4)"
    Time (mean ± σ):      12.4 ms ±   0.2 ms    [User: 9.2 ms, System: 3.9 ms]
    Range (min … max):    11.8 ms …  13.0 ms    214 runs

  Benchmark 3: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 256)"
    Time (mean ± σ):      12.6 ms ±   0.2 ms    [User: 9.5 ms, System: 3.8 ms]
    Range (min … max):    12.1 ms …  13.3 ms    209 runs

  Benchmark 4: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 4096)"
    Time (mean ± σ):      15.9 ms ±   0.2 ms    [User: 11.4 ms, System: 5.1 ms]
    Range (min … max):    15.2 ms …  16.4 ms    171 runs

  Benchmark 5: nix eval --raw --expr "let s = __concatStringsSep \"\" (__genList (_: \"c\") 256); in __toJSON (__genList (_: s) 65536)"
    Time (mean ± σ):      69.0 ms ±   0.9 ms    [User: 44.3 ms, System: 25.3 ms]
    Range (min … max):    67.2 ms …  70.9 ms    42 runs

0 files changed, 0 insertions, 0 deletions