Middle three digits: Difference between revisions

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 middleThreeDigits(-2002): Need odd and >= 3 digits
 middleThreeDigits(0): Need odd and >= 3 digits</pre>
+===Alternative Version===
+This longer version is more correct (works with T.min), gives a stronger typed output, and it tries to be faster avoiding conversions to string.
+<lang d>import std.stdio, std.traits, std.math, std.variant;
+/// Returns a string with the error, or the three digits.
+Algebraic!(string, char[3]) middleThreeDigits(T)(in T n)
+if (isIntegral!T) {
+    // Awkward code to face abs(T.min) when T is signed.
+    ulong ln;
+    static if (isSigned!T) {
+        if (n >= 0) {
+            ln = n;
+        } else {
+            if (n == T.min) {
+                ln = -(n + 1);
+                ln++;
+            } else {
+                ln = -n;
+            }
+        }
+    } else {
+        ln = n;
+    }
+    if (ln < 100)
+        return typeof(return)("n is too short.");
+    immutable uint digits = 1 + cast(uint)log10(ln);
+    if (digits % 2 == 0)
+        return typeof(return)("n must have an odd number of digits.");
+    // From the Reddit answer by "millstone".
+    int drop = (digits - 3) / 2;
+    while (drop-- > 0)
+        ln /= 10;
+    char[3] result = void;
+    result[2] = ln % 10 + '0';
+    ln /= 10;
+    result[1] = ln % 10 + '0';
+    ln /= 10;
+    result[0] = ln % 10 + '0';
+    return typeof(return)(result);
+}
+void main() {
+    immutable passing = [123, 12345, 1234567, 987654321, 10001,
+                         -10001, -123, -100, 100, -12345, -8765432];
+    foreach (n; passing) {
+        auto mtd = middleThreeDigits(n);
+        // A string result means it didn't pass.
+        assert(!mtd.peek!string);
+        writefln("middleThreeDigits(%d): %s", n, mtd);
+    }
+    writeln();
+    immutable failing = [1, 2, -1, -10, 2002, -2002, 0,
+, int.min, int.max];
+    foreach (n; failing) {
+        auto mtd = middleThreeDigits(n);
+        assert(mtd.peek!string);
+        writefln("middleThreeDigits(%d): %s", n, mtd);
+    }
+    writeln();
+    immutable long[] passingL = [123, 12345, 1234567, 987654321, 10001,
+                                 -10001, -123, -100, 100, -12345,
+                                 -8765432, long.min, long.max];
+    foreach (n; passingL) {
+        auto mtd = middleThreeDigits(n);
+        assert(!mtd.peek!string);
+        writefln("middleThreeDigits(%d): %s", n, mtd);
+    }
+    writeln();
+    immutable long[] failingL = [1, 2, -1, -10, 2002, -2002, 0, 15];
+    foreach (n; failingL) {
+        auto mtd = middleThreeDigits(n);
+        assert(mtd.peek!string);
+        writefln("middleThreeDigits(%d): %s", n, mtd);
+    }
+    writeln();
+    {
+        immutable n = short.min;
+        auto mtd = middleThreeDigits(n);
+        assert(!mtd.peek!string);
+        writefln("middleThreeDigits(cast(short)%d): %s", n, mtd);
+    }
+}</lang>
+{{out}}
+<pre>middleThreeDigits(123): 123
+middleThreeDigits(12345): 234
+middleThreeDigits(1234567): 345
+middleThreeDigits(987654321): 654
+middleThreeDigits(10001): 000
+middleThreeDigits(-10001): 000
+middleThreeDigits(-123): 123
+middleThreeDigits(-100): 100
+middleThreeDigits(100): 100
+middleThreeDigits(-12345): 234
+middleThreeDigits(-8765432): 654
+middleThreeDigits(1): n is too short.
+middleThreeDigits(2): n is too short.
+middleThreeDigits(-1): n is too short.
+middleThreeDigits(-10): n is too short.
+middleThreeDigits(2002): n must have an odd number of digits.
+middleThreeDigits(-2002): n must have an odd number of digits.
+middleThreeDigits(0): n is too short.
+middleThreeDigits(15): n is too short.
+middleThreeDigits(-2147483648): n must have an odd number of digits.
+middleThreeDigits(2147483647): n must have an odd number of digits.
+middleThreeDigits(123): 123
+middleThreeDigits(12345): 234
+middleThreeDigits(1234567): 345
+middleThreeDigits(987654321): 654
+middleThreeDigits(10001): 000
+middleThreeDigits(-10001): 000
+middleThreeDigits(-123): 123
+middleThreeDigits(-100): 100
+middleThreeDigits(100): 100
+middleThreeDigits(-12345): 234
+middleThreeDigits(-8765432): 654
+middleThreeDigits(-9223372036854775808): 368
+middleThreeDigits(9223372036854775807): 368
+middleThreeDigits(1): n is too short.
+middleThreeDigits(2): n is too short.
+middleThreeDigits(-1): n is too short.
+middleThreeDigits(-10): n is too short.
+middleThreeDigits(2002): n must have an odd number of digits.
+middleThreeDigits(-2002): n must have an odd number of digits.
+middleThreeDigits(0): n is too short.
+middleThreeDigits(15): n is too short.
+middleThreeDigits(cast(short)-32768): 276</pre>
 =={{header|OCaml}}==