var assert = require('assert')
var opcodes = require('./opcodes')

// https://github.com/feross/buffer/blob/master/index.js#L1127
function verifuint(value, max) {
  assert(typeof value === 'number', 'cannot write a non-number as a number')
  assert(value >= 0, 'specified a negative value for writing an unsigned value')
  assert(value <= max, 'value is larger than maximum value for type')
  assert(Math.floor(value) === value, 'value has a fractional component')
}

function pushDataSize(i) {
  return i < opcodes.OP_PUSHDATA1 ? 1
    : i < 0xff        ? 2
    : i < 0xffff      ? 3
    :                   5
}

function readPushDataInt(buffer, offset) {
  var opcode = buffer.readUInt8(offset)
  var number, size

  // ~6 bit
  if (opcode < opcodes.OP_PUSHDATA1) {
    number = opcode
    size = 1

  // 8 bit
  } else if (opcode === opcodes.OP_PUSHDATA1) {
    number = buffer.readUInt8(offset + 1)
    size = 2

  // 16 bit
  } else if (opcode === opcodes.OP_PUSHDATA2) {
    number = buffer.readUInt16LE(offset + 1)
    size = 3

  // 32 bit
  } else {
    assert.equal(opcode, opcodes.OP_PUSHDATA4, 'Unexpected opcode')

    number = buffer.readUInt32LE(offset + 1)
    size = 5

  }

  return {
    opcode: opcode,
    number: number,
    size: size
  }
}

function readUInt64LE(buffer, offset) {
  var a = buffer.readUInt32LE(offset)
  var b = buffer.readUInt32LE(offset + 4)
  b *= 0x100000000

  verifuint(b + a, 0x001fffffffffffff)

  return b + a
}

function readVarInt(buffer, offset) {
  var t = buffer.readUInt8(offset)
  var number, size

  // 8 bit
  if (t < 253) {
    number = t
    size = 1

  // 16 bit
  } else if (t < 254) {
    number = buffer.readUInt16LE(offset + 1)
    size = 3

  // 32 bit
  } else if (t < 255) {
    number = buffer.readUInt32LE(offset + 1)
    size = 5

  // 64 bit
  } else {
    number = readUInt64LE(buffer, offset + 1)
    size = 9
  }

  return {
    number: number,
    size: size
  }
}

function writePushDataInt(buffer, number, offset) {
  var size = pushDataSize(number)

  // ~6 bit
  if (size === 1) {
    buffer.writeUInt8(number, offset)

  // 8 bit
  } else if (size === 2) {
    buffer.writeUInt8(opcodes.OP_PUSHDATA1, offset)
    buffer.writeUInt8(number, offset + 1)

  // 16 bit
  } else if (size === 3) {
    buffer.writeUInt8(opcodes.OP_PUSHDATA2, offset)
    buffer.writeUInt16LE(number, offset + 1)

  // 32 bit
  } else {
    buffer.writeUInt8(opcodes.OP_PUSHDATA4, offset)
    buffer.writeUInt32LE(number, offset + 1)

  }

  return size
}

function writeUInt64LE(buffer, value, offset) {
  verifuint(value, 0x001fffffffffffff)

  buffer.writeInt32LE(value & -1, offset)
  buffer.writeUInt32LE(Math.floor(value / 0x100000000), offset + 4)
}

function varIntSize(i) {
  return i < 253      ? 1
    : i < 0x10000     ? 3
    : i < 0x100000000 ? 5
    :                   9
}

function writeVarInt(buffer, number, offset) {
  var size = varIntSize(number)

  // 8 bit
  if (size === 1) {
    buffer.writeUInt8(number, offset)

  // 16 bit
  } else if (size === 3) {
    buffer.writeUInt8(253, offset)
    buffer.writeUInt16LE(number, offset + 1)

  // 32 bit
  } else if (size === 5) {
    buffer.writeUInt8(254, offset)
    buffer.writeUInt32LE(number, offset + 1)

  // 64 bit
  } else {
    buffer.writeUInt8(255, offset)
    writeUInt64LE(buffer, number, offset + 1)
  }

  return size
}

module.exports = {
  pushDataSize: pushDataSize,
  readPushDataInt: readPushDataInt,
  readUInt64LE: readUInt64LE,
  readVarInt: readVarInt,
  varIntSize: varIntSize,
  writePushDataInt: writePushDataInt,
  writeUInt64LE: writeUInt64LE,
  writeVarInt: writeVarInt
}