package proc

import (
	"fmt"
	"io"
)

// MemoryReader is like io.ReaderAt, but the offset is a uintptr so that it
// can address all of 64-bit memory.
// Redundant with memoryReadWriter but more easily suited to working with
// the standard io package.
type MemoryReader interface {
	// ReadMemory is just like io.ReaderAt.ReadAt.
	ReadMemory(buf []byte, addr uintptr) (n int, err error)
}

// A SplicedMemory represents a memory space formed from multiple regions,
// each of which may override previously regions. For example, in the following
// core, the program text was loaded at 0x400000:
// Start               End                 Page Offset
// 0x0000000000400000  0x000000000044f000  0x0000000000000000
// but then it's partially overwritten with an RW mapping whose data is stored
// in the core file:
// Type           Offset             VirtAddr           PhysAddr
//                FileSiz            MemSiz              Flags  Align
// LOAD           0x0000000000004000 0x000000000049a000 0x0000000000000000
//                0x0000000000002000 0x0000000000002000  RW     1000
// This can be represented in a SplicedMemory by adding the original region,
// then putting the RW mapping on top of it.
type SplicedMemory struct {
	readers []readerEntry
}

type readerEntry struct {
	offset uintptr
	length uintptr
	reader MemoryReader
}

// Add adds a new region to the SplicedMemory, which may override existing regions.
func (r *SplicedMemory) Add(reader MemoryReader, off, length uintptr) {
	if length == 0 {
		return
	}
	end := off + length - 1
	newReaders := make([]readerEntry, 0, len(r.readers))
	add := func(e readerEntry) {
		if e.length == 0 {
			return
		}
		newReaders = append(newReaders, e)
	}
	inserted := false
	// Walk through the list of regions, fixing up any that overlap and inserting the new one.
	for _, entry := range r.readers {
		entryEnd := entry.offset + entry.length - 1
		switch {
		case entryEnd < off:
			// Entry is completely before the new region.
			add(entry)
		case end < entry.offset:
			// Entry is completely after the new region.
			if !inserted {
				add(readerEntry{off, length, reader})
				inserted = true
			}
			add(entry)
		case off <= entry.offset && entryEnd <= end:
			// Entry is completely overwritten by the new region. Drop.
		case entry.offset < off && entryEnd <= end:
			// New region overwrites the end of the entry.
			entry.length = off - entry.offset
			add(entry)
		case off <= entry.offset && end < entryEnd:
			// New reader overwrites the beginning of the entry.
			if !inserted {
				add(readerEntry{off, length, reader})
				inserted = true
			}
			overlap := entry.offset - off
			entry.offset += overlap
			entry.length -= overlap
			add(entry)
		case entry.offset < off && end < entryEnd:
			// New region punches a hole in the entry. Split it in two and put the new region in the middle.
			add(readerEntry{entry.offset, off - entry.offset, entry.reader})
			add(readerEntry{off, length, reader})
			add(readerEntry{end + 1, entryEnd - end, entry.reader})
			inserted = true
		default:
			panic(fmt.Sprintf("Unhandled case: existing entry is %v len %v, new is %v len %v", entry.offset, entry.length, off, length))
		}
	}
	if !inserted {
		newReaders = append(newReaders, readerEntry{off, length, reader})
	}
	r.readers = newReaders
}

// ReadMemory implements MemoryReader.ReadMemory.
func (r *SplicedMemory) ReadMemory(buf []byte, addr uintptr) (n int, err error) {
	started := false
	for _, entry := range r.readers {
		if entry.offset+entry.length < addr {
			if !started {
				continue
			}
			return n, fmt.Errorf("hit unmapped area at %v after %v bytes", addr, n)
		}

		// Don't go past the region.
		pb := buf
		if addr+uintptr(len(buf)) > entry.offset+entry.length {
			pb = pb[:entry.offset+entry.length-addr]
		}
		pn, err := entry.reader.ReadMemory(pb, addr)
		n += pn
		if err != nil || pn != len(pb) {
			return n, err
		}
		buf = buf[pn:]
		addr += uintptr(pn)
		if len(buf) == 0 {
			// Done, don't bother scanning the rest.
			return n, nil
		}
	}
	if n == 0 {
		return 0, fmt.Errorf("offset %v did not match any regions", addr)
	}
	return n, nil
}

// OffsetReaderAt wraps a ReaderAt into a MemoryReader, subtracting a fixed
// offset from the address. This is useful to represent a mapping in an address
// space. For example, if program text is mapped in at 0x400000, an
// OffsetReaderAt with offset 0x400000 can be wrapped around file.Open(program)
// to return the results of a read in that part of the address space.
type OffsetReaderAt struct {
	reader io.ReaderAt
	offset uintptr
}

func (r *OffsetReaderAt) ReadMemory(buf []byte, addr uintptr) (n int, err error) {
	return r.reader.ReadAt(buf, int64(addr-r.offset))
}