fix base(b, big(0), 0) == "0"

Ref #22133
(cherry picked from commit 994f42c)

base/gmp.jl

@@ -578,26 +578,21 @@ oct(n::BigInt, pad::Int) = base( 8, n, pad)
 dec(n::BigInt, pad::Int) = base(10, n, pad)
 hex(n::BigInt, pad::Int) = base(16, n, pad)
 
-function base(b::Integer, n::BigInt)
+function base(b::Integer, n::BigInt, pad::Integer=1)
+    b < 0 && return base(Int(b), n, pad, (b>0) & (n.size<0))
     2 <= b <= 62 || throw(ArgumentError("base must be 2 ≤ base ≤ 62, got $b"))
-    nd = ndigits(n, b)
-    str = Base._string_n(n < 0 ? nd+1 : nd)
-    ccall((:__gmpz_get_str,:libgmp), Ptr{UInt8}, (Ptr{UInt8}, Cint, Ptr{BigInt}), str, b, &n)
-    return str
-end
-
-function base(b::Integer, n::BigInt, pad::Integer)
-    s = base(b, n)
-    buf = IOBuffer()
-    if n < 0
-        s = s[2:end]
-        write(buf, '-')
+    nd1 = ndigits(n, b)
+    nd  = max(nd1, pad)
+    str = Base._string_n(nd + isneg(n) + 1) # +1 for final '\0'
+    ptr = pointer(str)
+    ccall((:__gmpz_get_str,:libgmp), Ptr{UInt8}, (Ptr{UInt8}, Cint, Ref{BigInt}), ptr + nd - nd1, b, n)
+    for i = (0:nd-nd1-1) + isneg(n)
+        unsafe_store!(ptr+i, '0' % UInt8)
     end
-    for i in 1:pad-sizeof(s) # `s` is known to be ASCII, and `length` is slower
-        write(buf, '0')
-    end
-    write(buf, s)
-    String(buf)
+    isneg(n) && unsafe_store!(ptr, '-' % UInt8)
+    str.len -= 1 # final '\0'
+    iszero(n) && pad < 1 && (str.len -= 1)
+    str
 end
 
 function ndigits0z(x::BigInt, b::Integer=10)

test/bigint.jl

@@ -333,6 +333,12 @@ let padding = 4, low = big(4), high = big(2^20)
     @test hex(-high, padding) == "-100000"
 end
 
+# respect 0-padding on big(0)
+for f in (bin, oct, dec, hex)
+    @test f(big(0), 0) == ""
+end
+@test base(rand(2:62), big(0), 0) == ""
+
 @test isqrt(big(4)) == 2
 @test isqrt(big(5)) == 2