扩展切片的大小以防止切片边界超出范围错误

Question

I have written the following:

func main() {
    //inside main        
        fileInputBytes, err := ioutil.ReadFile("/tmp/test")
        byteSize2 := len(fileInputBytes)

        var inputFileByteSlice = fileInputBytes[0:]
        var numberOfIndexes = math.Floor(float64(byteSize / indexingOffset))

        for i := 1; i <= int(numberOfIndexes); i++ {
            // adding i to the indexer insures that we use lookahed to ignore previously inserted indexing values
            var v int = (i * indexingOffset) + i
            Insert(&inputFileByteSlice, v+i, indexingByteValue)
            fmt.Println(i)
        }
    }
    //outside main
    //variation of https://blog.golang.org/slices with pointers and such
        func Insert(slice *[]byte, index int, value byte) {
            // Grow the slice by one element.
            (*slice) = (*slice)[0 : len(*slice)+1]
            // Use copy to move the upper part of the slice out of the way and open a hole.
            copy((*slice)[index+1:], (*slice)[index:])
            // Store the new value.
            (*slice)[index] = value
            // Return the result.
        }

The slice bounds out of range error is getting on my nerves. The length of the slice grows outside of the size and overflows, the reason I don't understand is that I thought the call to 'grow' the slice by one element(before copy) will dynamically allocate more space. Since that is not the case, can anyone offer me a better suggestion?

Answer 1

First of all, a slice is already a reference type. So you don't need to pass its pointer around if you are not going to change its capacity. So your main can be simplified as:

func main() {
    fileInputBytes, err := ioutil.ReadFile("/tmp/test")
    byteSize2 := len(fileInputBytes)

    // No need to use pointer to slice. If you want a brand new slice
    // that does not affect the original slice values, use copy()
    inputFileByteArray := fileInputBytes
    var numberOfIndexes = math.Floor(float64(byteSize / indexingOffset))

    for i := 1; i <= int(numberOfIndexes); i++ {
        var v int = (i * indexingOffset) + i

        // Insert needs to return the newly updated slice reference
        // which should be assigned in each iteration.
        inputFileByteArray = Insert(inputFileByteArray, v+i, indexingByteValue)
        fmt.Println(i)
    }
}

Then, the Insert function can be simplified simply by using append along with copy and returning the newly created slice:

func Insert(slice []byte, index int, value byte) []byte {
    if index >= len(slice) {
        // add to the end of slice in case of index >= len(slice)
        return append(slice, value)
    }
    tmp := make([]byte, len(slice[:index + 1]))
    copy(tmp, slice[:index])
    tmp[index] = value
    return append(tmp, slice[index:]...)
}

This may not be the best implementation but it is simple enough. Example usage at: https://play.golang.org/p/Nuq4RX9XQD