package wasm

import (
	"fmt"
)

const (
	// MemoryPageSize is the unit of memory length in WebAssembly,
	// and is defined as 2^16 = 65536.
	// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memory-instances%E2%91%A0
	MemoryPageSize = uint32(65536)
	// MemoryPageSizeInBits satisfies the relation: "1 << MemoryPageSizeInBits == MemoryPageSize".
	MemoryPageSizeInBits = 16
)

// MemoryPagesToBytesNum converts the given pages into the number of bytes contained in these pages.
func MemoryPagesToBytesNum(pages uint32) (bytesNum uint64) {
	return uint64(pages) << MemoryPageSizeInBits
}

// PagesToUnitOfBytes converts the pages to a human-readable form similar to what's specified. Ex. 1 -> "64Ki"
//
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memory-instances%E2%91%A0
func PagesToUnitOfBytes(pages uint32) string {
	k := pages * 64
	if k < 1024 {
		return fmt.Sprintf("%d Ki", k)
	}
	m := k / 1024
	if m < 1024 {
		return fmt.Sprintf("%d Mi", m)
	}
	g := m / 1024
	if g < 1024 {
		return fmt.Sprintf("%d Gi", g)
	}
	return fmt.Sprintf("%d Ti", g/1024)
}

// Below are raw functions used to implement the api.Memory API:

// memoryBytesNumToPages converts the given number of bytes into the number of pages.
func memoryBytesNumToPages(bytesNum uint64) (pages uint32) {
	return uint32(bytesNum >> MemoryPageSizeInBits)
}