run-length encoding

gistfile1.lua

-- Use literal zero for escape before compression data
local esc = 0

-- Run-length-encode an array of ints.
function encode(ints)
    local res, i, ct = {}, 1, 1  -- result, index, count
    while i <= #ints do
        local v = ints[i]
        while ints[i+1] == v do
            ct = ct + 1
            i = i + 1
        end
        
        -- if there's benefit in reducing a series
        if ct > 3 or (ct >= 2 and v == esc) then
            res[#res+1] = 0
            res[#res+1] = ct
            res[#res+1] = v
        -- literal zeroes must be escaped
        elseif v == esc then
            for n=1,ct do
                res[#res+1] = esc
                res[#res+1] = esc
            end
        -- leave other content as-is
        else
            for n=1,ct do
                res[#res+1] = v
            end
        end
        ct = 1
        i = i + 1
    end
    return res
end

-- Decode an array of ints from encode().
function decode(ints)
    local res = {}
    local i = 1
    while i <= #ints do
        local v = ints[i]

        -- check for escaped 0s and series markers after escapes
        if v == esc then
            if ints[i+1] == esc then
                res[#res+1] = esc     -- escaped 0 literal
                i = i + 2
            else
                local count, value = ints[i+1], ints[i+2]
                for n=1,count do res[#res+1] = value end
                i = i + 3
            end
        else
            res[#res+1] = v
            i = i + 1
        end
    end
    return res
end