How to convert a UPC-E barcode to a UPC-A barcode?

Question

What is the algorithm for converting a zero-suppressed, eight-digit GTIN-12 identifier (represented as a UPC-E barcode) into the full, twelve-digit version as shown in a UPC

Answer 1

The algorithm for converting a GTIN-12 identifier between UPC-E and UPC-A representation can be most clearly seen from the following pattern mapping:

SabcdeNX ⟺ SabN0000cdeX  :  0≤N≤2
Sabcde3X ⟺ Sabc00000deX
Sabcde4X ⟺ Sabcd00000eX
SabcdeNX ⟺ Sabcde0000NX  :  5≤N≤9

In the above S is the number system (either 0 or 1) and X is the check digit.

In pseudo-code it looks like this:

Input:  A valid eight-digit UPC-E: Assigned to E[]. 
Output: PASS: Twelve-digit UPC-A representing the UPC-E.
        FAIL: Reason.

if E[0] != {0-1} then FAIL: Invalid number system.

if E[6] == {0-2} then PASS: E[0..2] . E[6] . "0000"  . E[3..5] . E[7]
if E[6] == "3"   then PASS: E[0..3] .        "00000" . E[4..5] . E[7]
if E[6] == "4"   then PASS: E[0..4] .        "00000" . E[5]    . E[7]    
if E[6] == {5-9} then PASS: E[0..5] .        "0000"  . E[6]    . E[7]

Answer 2

This duplicates the algorithm in @Terry Burton's answer, written in Java.

switch (data.charAt(6)) {
    case '0':
    case '1':
    case '2': {
        data = data.substring(0, 3) + "0000" + data.charAt(6) + data.substring(3, 6) + data.charAt(7);
        break;
    }
    case '3': {
        data = data.substring(0, 4) + "00000" + data.substring(4, 5) + data.charAt(7);
        break;
    }
    case '4': {
        data = data.substring(0, 5) + "00000" + data.charAt(5) + data.charAt(7);
        break;
    }
    case '5':
    case '6':
    case '7':
    case '8':
    case '9': {
        data = data.substring(0, 6) + "0000" + data.charAt(6) + data.charAt(7);
        break;
    }
}