AoC19/d04.lua

n_begin, n_end = string.match(arg[1], "(%d+)-(%d+)")

--[[
t_begin = {}
t_end = {}
for c in n_begin:gmatch(".") do
    table.insert(t_begin, tonumber(c))
end
for c in n_end:gmatch(".") do
    table.insert(t_end, tonumber(c))
end

-- Minimum possible value
highest = 0
for i, x in ipairs(t_begin) do
    if x < highest then
        t_begin[i] = highest
    elseif x > highest then
        highest = x
    end
end

function compare(tbl1, tbl2)
    for i, x in ipairs(tbl1) do
        local v = x - tbl2[i]
        if v ~= 0 then
            return v
        end
    end
end

num = {table.unpack(t_begin)}

part1 = 0
while compare(num, t_end) < 0 do
    for i = 1, #num do
        if num[i] < t_end[i] then
            if num[i] < num[i + 1] then
                num[i] = num[i] + 1
                part1 = part1 + 1
            end
        else
        end
    end
end

print(part1)

for _, x in ipairs(num) do
    io.stdout:write(x)
end
io.stdout:write("\n")
]]--

function is_valid(num)
    local str = tostring(num)
    local lastchar = nil
    local inarow = 0
    local gotmatch = false
    for char = 1, #str do
        if lastchar and str:byte(char) < lastchar then
            return false
        end
        if str:byte(char) == lastchar then
            inarow = inarow + 1
            --gotmatch = true
        else
            if inarow == 1 then
                gotmatch = true
            end
            inarow = 0
        end
        lastchar = str:byte(char)
    end
    if inarow == 1 then
        gotmatch = true
    end
    return gotmatch
end

possibilities = 0
for i = tonumber(n_begin), tonumber(n_end) do
    if is_valid(i) then
        possibilities = possibilities + 1
    end
end
print(possibilities)