--[[
    Titty has two main design goals:
    1. Should look natural from the outside, Lua code's PoV, and
    2. Should compile to Lua code that also appears natural from within
    The second is less important than the first.
]]

local inspect = require"inspect".inspect

local function codepoints(str)
    local it, state, v1, v2 = utf8.codes(str)
    local idx = 0
    return function()
        if not it then return nil end
        
        v1, v2 = it(state, v1)
        if v1 then
            local r = idx
            idx = idx + 1
            return r, v2
        else
            it = nil
        end
    end
end

local function is_digit(cp)
    return cp and (cp >= 48 and cp <= 57)
end

local function is_ident_start(cp)
    return cp and ((cp >= 65 and cp <= 90) or (cp >= 97 and cp <= 122) or cp == 95)
end

local function is_ident_nonstart(cp)
    return cp and (is_ident_start(cp) or is_digit(cp))
end

local function forall(tbl, f, startI)
    startI = startI or 1
    
    for k, v in pairs(tbl) do
        if not f(v) then
            return false
        end
    end
    return true
end

local function is_whitespace(cp)
    return cp and (cp == 32 or cp == 10 or cp == 9)
end

local function clear(tbl)
    for k in pairs(tbl) do
        tbl[k] = nil
    end
end

local function lex(cpgetraw)
    local row, column, idx = 0, 0, 0
    local pull
    local function cpget()
        if pull then
            local ret = pull
            pull = nil
            return ret
        else
            local cp
            idx, cp = cpgetraw()
            column = column + 1
            if cp == 10 then
                column = 0
                row = row + 1
            end
            return cp
        end
    end
    
    local buf = {}
    
    return function()
        local cp
        
        while true do
            cp = cpget()
            
            if not is_whitespace(cp) then
                break
            end
        end
        
        if not cp then
            return nil
        end
        
        local rowStart, columnStart = row, column
        
        if cp == 40 then
            return "(", "(", rowStart, columnStart
        elseif cp == 41 then
            return ")", ")", rowStart, columnStart
        elseif cp == 123 then
            return "{", "{", rowStart, columnStart
        elseif cp == 125 then
            return "}", "}", rowStart, columnStart
        elseif cp == 44 then
            return ",", ",", rowStart, columnStart
        elseif cp == 46 then
            return ".", ".", rowStart, columnStart
        elseif cp == 58 then
            return ":", ":", rowStart, columnStart
        elseif cp == 61 then
            local after = cpget()
            if after == 61 then
                return "==", "==", rowStart, columnStart
            else
                pull = after
            end
            return "=", "=", rowStart, columnStart
        elseif cp == 43 then
            return "+", "+", rowStart, columnStart
        elseif cp == 45 then
            return "-", "-", rowStart, columnStart
        elseif cp == 42 then
            local after = cpget()
            if after == 42 then
                return "**", "**", rowStart, columnStart
            else
                pull = after
            end
            return "*", "*", rowStart, columnStart
        elseif cp == 47 then
            return "/", "/", rowStart, columnStart
        elseif cp == 37 then
            return "%", "%", rowStart, columnStart
        elseif cp == 35 then
            return "#", "#", rowStart, columnStart
        elseif cp == 91 then
            return "[", "[", rowStart, columnStart
        elseif cp == 93 then
            return "]", "]", rowStart, columnStart
        elseif cp == 39 then
            while true do
                cp = cpget()
                
                if cp ~= 39 then
                    buf[#buf + 1] = cp
                else
                    break
                end
            end
            
            local ret = utf8.char(table.unpack(buf))
            
            clear(buf)
            
            return "string", ret, rowStart, columnStart
        elseif is_ident_start(cp) then
            buf[1] = cp
            
            while true do
                cp = cpget()
                
                if is_ident_nonstart(cp) then
                    buf[#buf + 1] = cp
                else
                    pull = cp
                    break
                end
            end
            
            local ret = utf8.char(table.unpack(buf))
            
            clear(buf)
            
            if ret == "func" or ret == "end" or ret == "if"
                or ret == "while" or ret == "do" or ret == "then"
                or ret == "elseif" or ret == "type" or ret == "interf"
                or ret == "let" or ret == "return" or ret == "else"
                or ret == "for" or ret == "import" or ret == "constr"
                or ret == "loop" or ret == "break" or ret == "nil" then
                
                return ret, ret, rowStart, columnStart
            end
            
            return "id", ret, rowStart, columnStart
        elseif is_digit(cp) then
            buf[1] = cp
            
            local dotFound = false
            
            while true do
                cp = cpget()
                if is_digit(cp) then
                    buf[#buf + 1] = cp
                elseif cp == 46 and not dotFound then
                    dotFound = true
                    buf[#buf + 1] = cp
                else
                    pull = cp
                    break
                end
            end
            
            local ret = utf8.char(table.unpack(buf))
            clear(buf)
            return "num", ret, rowStart, columnStart
        else
            error(string.format("%i:%i unknown character %q (code point %i)", row, column, utf8.char(cp), cp))
        end
    end
end

return {lex = lex, codepoints = codepoints}