// https://github.com/lyngklip/structjs
"use strict"
const rechk = /^([<>])?(([1-9]\d*)?([xcbB?hHiIfdsp]))*$/
const refmt = /([1-9]\d*)?([xcbB?hHiIfdsp])/g
const str = (v,o,c) => String.fromCharCode(
    ...new Uint8Array(v.buffer, v.byteOffset + o, c))
const rts = (v,o,c,s) => new Uint8Array(v.buffer, v.byteOffset + o, c)
    .set(s.split('').map(str => str.charCodeAt(0)))
const pst = (v,o,c) => str(v, o + 1, Math.min(v.getUint8(o), c - 1))
const tsp = (v,o,c,s) => { v.setUint8(o, s.length); rts(v, o + 1, c - 1, s) }
const lut = le => ({
    x: c=>[1,c,0],
    c: c=>[c,1,o=>({u:v=>str(v, o, 1)      , p:(v,c)=>rts(v, o, 1, c)     })],
    '?': c=>[c,1,o=>({u:v=>Boolean(v.getUint8(o)),p:(v,B)=>v.setUint8(o,B)})],
    b: c=>[c,1,o=>({u:v=>v.getInt8(   o   ), p:(v,b)=>v.setInt8(   o,b   )})],
    B: c=>[c,1,o=>({u:v=>v.getUint8(  o   ), p:(v,B)=>v.setUint8(  o,B   )})],
    h: c=>[c,2,o=>({u:v=>v.getInt16(  o,le), p:(v,h)=>v.setInt16(  o,h,le)})],
    H: c=>[c,2,o=>({u:v=>v.getUint16( o,le), p:(v,H)=>v.setUint16( o,H,le)})],
    i: c=>[c,4,o=>({u:v=>v.getInt32(  o,le), p:(v,i)=>v.setInt32(  o,i,le)})],
    I: c=>[c,4,o=>({u:v=>v.getUint32( o,le), p:(v,I)=>v.setUint32( o,I,le)})],
    f: c=>[c,4,o=>({u:v=>v.getFloat32(o,le), p:(v,f)=>v.setFloat32(o,f,le)})],
    d: c=>[c,8,o=>({u:v=>v.getFloat64(o,le), p:(v,d)=>v.setFloat64(o,d,le)})],
    s: c=>[1,c,o=>({u:v=>str(v,o,c), p:(v,s)=>rts(v,o,c,s.slice(0,c    ) )})],
    p: c=>[1,c,o=>({u:v=>pst(v,o,c), p:(v,s)=>tsp(v,o,c,s.slice(0,c - 1) )})]
})
const errbuf = new RangeError("Structure larger than remaining buffer")
const errval = new RangeError("Not enough values for structure")
const struct = format => {
    let fns = [], size = 0, m = rechk.exec(format)
    if (!m) { throw new RangeError("Invalid format string") }
    const t = lut('<' === m[1]), lu = (n, c) => t[c](n ? parseInt(n, 10) : 1)
    while ((m = refmt.exec(format))) { ((r, s, f) => {
        for (let i = 0; i < r; ++i, size += s) { if (f) {fns.push(f(size))} }
    })(...lu(...m.slice(1)))}
    const unpack_from = (arrb, offs) => {
        if (arrb.byteLength < (offs|0) + size) { throw errbuf }
        let v = new DataView(arrb, offs|0)
        return fns.map(f => f.u(v))
    }
    const pack_into = (arrb, offs, ...values) => {
        if (values.length < fns.length) { throw errval }
        if (arrb.byteLength < offs + size) { throw errbuf }
        const v = new DataView(arrb, offs)
        new Uint8Array(arrb, offs, size).fill(0)
        fns.forEach((f, i) => f.p(v, values[i]))
    }
    const pack = (...values) => {
        let b = new ArrayBuffer(size)
        pack_into(b, 0, ...values)
        return b
    }
    const unpack = arrb => unpack_from(arrb, 0)
    function* iter_unpack(arrb) { 
        for (let offs = 0; offs + size <= arrb.byteLength; offs += size) {
            yield unpack_from(arrb, offs);
        }
    }
    return Object.freeze({
        unpack, pack, unpack_from, pack_into, iter_unpack, format, size})
}