CODEWITHSUNDEEP
javascriptpython
majidarif
majidarif

Reputation: 20075

Decompression Routine In Javascript to Python

I'm trying to rewrite a javascript file decompression routine to python. But I can't seem to get it right. The function always returns None on python.

Here is the original JS:

var c = new Uint8Array(b),
    d = 0,
    e = new Uint32Array(4096),
    f = new Uint32Array(4096),
    g = 256,
    h = a.length,
    k = 0,
    l = 1,
    m = 0,
    n = 1;
c[d++] = a[0];
for (var r = 1;; r++) {
    n = r + (r >> 1);
    if (n + 1 >= h) break;
    var m = a[n + 1],
        n = a[n],
        p = r & 1 ? m << 4 | n >> 4 : (m & 15) << 8 | n;
    if (p < g)
        if (256 > p) m = d, n = 1, c[d++] = p;
        else
            for (var m = d, n = f[p], p = e[p], q = p + n; p < q;) c[d++] = c[p++];
    else if (p == g) {
        m = d;
        n = l + 1;
        p = k;
        for (q = k + l; p < q;) c[d++] = c[p++];
        c[d++] = c[k]
    } else break;
    e[g] = k;
    f[g++] = l + 1;
    k = m;
    l = n;
    g = 4096 <= g ? 256 : g
}
return d == b ? c : null

And here is my python implementation, what did I do wrong? (Improved with @le_m answer but still returning None.)

c = [0] * b
d = 0
e = [0] * 4096
f = [0] * 4096
g = 256
h = len(a)
k = 0
l = 1
m = 0
n = 1

c[d] = a[0]
d += 1

r = 1
while True:
    n = r + (r >> 1)
    if (n + 1) >= h:
        break
    m = a[n + 1]
    n = a[n]
    p = (m << 4 | n >> 4) if r & 1 else ((m & 15) << 8 | n)
    if (p < g):
        if (256 > p):
            m = d
            n = 1
            c[d] = p
            d += 1
        else:
            m = d
            n = f[p]
            p = e[p]
            q = p + n
            while p < q:
                c[d] = c[p]
                d += 1
                p += 1
    elif p == g:
        m = d
        n = 1 + 1
        p = k

        q = k + l
        while p < q:
            c[d] = c[p]
            d += 1
            p += 1
    else:
        break

    e[g] = k
    f[g] = l + 1
    g += 1
    k = m
    l = n
    g = 256 if 4096 <= g else g
    r += 1

Upvotes: 0

Views: 39

Answers (1)

le_m
le_m

Reputation: 20248

Just from glancing at the code, I spot the following differences:

  • new Uint8Array(b) translates to [0] * b as the typed array is initialized with zeros. Same holds for the other typed arrays.
  • c[d++] = a[0] translates to c[d] = a[0] followed by d += 1, not the other way round. Same holds for the other post-increments, too.
  • The loop counter r += 1 should go to the end of the loop body or should be initialized with 0 instead.

I recommend stepping through both implementations step by step with a debugger (your browser's developer console for JavaScript) and compare the variable values.

Upvotes: 2

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