Add tinyfs.SliceBlockDev that will "partition" a block dev

This is useful to create a fixed-size BlockDevice from machine.Flash,
which can vary when the firmware changes (and might require a format
each time it crosses a page boundary).

Tested with littlefs, it seems to be working fine.

Author: aykevl

slice.go

@@ -0,0 +1,75 @@
+package tinyfs
+
+import (
+	"errors"
+	"math/bits"
+)
+
+var (
+	ErrInvalidBlockSize = errors.New("tinyfs: write/erase block size too big or not aligned")
+	ErrDeviceTooSmall   = errors.New("tinyfs: requested device size is smaller than underlying device")
+)
+
+type blockDevOffset struct {
+	dev    BlockDevice
+	offset int64
+}
+
+// Create a new virtual BlockDevice that is the upper part of the given full
+// BlockDevice, with the given size. This can be useful for example to use only
+// a fixed size machine.Flash object that won't change on firmware changes.
+func SliceBlockDev(full BlockDevice, size int64) (BlockDevice, error) {
+	if bits.OnesCount16(uint16(full.WriteBlockSize())) != 1 || bits.OnesCount16(uint16(full.EraseBlockSize())) != 1 {
+		// Check that the write/erase block size is a power of two.
+		// This is normally the case, and makes the following calculations
+		// easier.
+		return nil, ErrInvalidBlockSize
+	}
+
+	// Check whether the requested size can be used (is a multiple of the block
+	// size).
+	blockSize := max(full.EraseBlockSize(), full.WriteBlockSize())
+	if size/blockSize*blockSize != size {
+		return nil, ErrInvalidBlockSize
+	}
+
+	fullSize := full.Size()
+	if fullSize < size {
+		// Not big enough to provide the expected device size.
+		return nil, ErrDeviceTooSmall
+	}
+	if fullSize == size {
+		// Shortcut: the requested size is exactly big enough.
+		return full, nil
+	}
+	offset := fullSize - size
+	return &blockDevOffset{
+		dev:    full,
+		offset: offset,
+	}, nil
+}
+
+func (b *blockDevOffset) ReadAt(buf []byte, offset int64) (n int, err error) {
+	return b.dev.ReadAt(buf, offset+b.offset)
+}
+
+func (b *blockDevOffset) WriteAt(buf []byte, offset int64) (n int, err error) {
+	return b.dev.WriteAt(buf, offset+b.offset)
+}
+
+func (b *blockDevOffset) Size() int64 {
+	return b.dev.Size() - b.offset
+}
+
+func (b *blockDevOffset) EraseBlockSize() int64 {
+	return b.dev.EraseBlockSize()
+}
+
+func (b *blockDevOffset) WriteBlockSize() int64 {
+	return b.dev.WriteBlockSize()
+}
+
+func (b *blockDevOffset) EraseBlocks(start, len int64) error {
+	blockOffset := b.offset / b.EraseBlockSize()
+	return b.dev.EraseBlocks(start+blockOffset, len)
+}