benchmark_thing.lua

local ffi = require 'ffi'
local C = ffi.C
ffi.cdef[[
typedef void FILE;

FILE * fopen ( const char * filename, const char * mode );
char * fgets ( char * str, int num, FILE * stream );
int fclose ( FILE * stream );
]]

local f = C.fopen('ngrams.tsv', 'r')
local bufsiz = 512
local buf = ffi.new('char[?]', bufsiz)

local sum_by_key = ffi.new('int[?]', 2009)
ffi.fill(sum_by_key, ffi.sizeof(sum_by_key))

local tab = string.byte('\t')
local zero = string.byte('0')
local function process(b)
    local i = 0
    local k, v = 0, 0

    while b[i] ~= tab do
        i = i + 1
    end
    i = i + 1
    while b[i] ~= tab do
        k = k*10 + b[i] - zero
        i = i + 1
    end
    i = i + 1
    while b[i] ~= tab do
        v = v*10 + b[i] - zero
        i = i + 1
    end
    return k, v
end
while C.fgets(buf, bufsiz, f) == buf do
    local k, v = process(buf)
    sum_by_key[k] = sum_by_key[k] + v
end

local k, v = -1, -1
for i=0,2009-1 do
    local val = sum_by_key[i]
    if val > v then
        k = i
        v = val
    end
end
C.fclose(f)

if k ~= 2006 or v ~= 22569013 then
    error('bad result '..k..' '..v..' sum_by_key[2006] = '..sum_by_key[2006])
end