Userland: Add a "seq" utility

It's useful with the newly added for loops in Shell.

SerenityOS's printf() doesn't seem to print decimals for
doubles correctly, so this doesn't print the right output for
fractional numbers yet.

Userland/seq.cpp

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2020, Nico Weber <thakis@chromium.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/StdLibExtras.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+double get_double(const char* name, const char* d_string, int* number_of_decimals)
+{
+    char* end;
+    double d = strtod(d_string, &end);
+    if (d == 0 && end == d_string) {
+        fprintf(stderr, "%s: invalid double vallue \"%s\"\n", name, d_string);
+        exit(1);
+    }
+    if (char* dot = strchr(d_string, '.'))
+        *number_of_decimals = strlen(dot + 1);
+    else
+        *number_of_decimals = 0;
+    return d;
+}
+
+int main(int argc, const char* argv[])
+{
+    if (pledge("stdio", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+    if (unveil(nullptr, nullptr) < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    double start = 1, step = 1, end = 1;
+    int number_of_start_decimals = 0, number_of_step_decimals = 0, number_of_end_decimals = 0;
+    switch (argc) {
+    case 2:
+        end = get_double(argv[0], argv[1], &number_of_end_decimals);
+        break;
+    case 3:
+        start = get_double(argv[0], argv[1], &number_of_start_decimals);
+        end = get_double(argv[0], argv[2], &number_of_end_decimals);
+        break;
+    case 4:
+        start = get_double(argv[0], argv[1], &number_of_start_decimals);
+        step = get_double(argv[0], argv[2], &number_of_step_decimals);
+        end = get_double(argv[0], argv[3], &number_of_end_decimals);
+        break;
+    default:
+        fprintf(stderr, "%s: unexpected number of arguments\n", argv[0]);
+        return 1;
+    }
+
+    if (step == 0) {
+        fprintf(stderr, "%s: increment must not be 0\n", argv[0]);
+        return 1;
+    }
+
+#if 0
+    // FIXME: Check for NaN once math.h has isnan().
+    if (isnan(start) || isnan(step) || isnan(end)) {
+        fprintf(stderr, "%s: start, step, and end must not be NaN\n", argv[0]);
+        return 1;
+    }
+#endif
+
+    int number_of_decimals = max(number_of_start_decimals, max(number_of_step_decimals, number_of_end_decimals));
+
+    int n = (end - start) / step;
+    double d = start;
+    for (int i = 0; i <= n; ++i) {
+        char buf[40];
+        sprintf(buf, "%f", d); // FIXME: Serenity's printf() doesn't seem to handle %f correctly: For `seq 1 0.1 2` this always prints "1.0" as `d` goes from 1.0 to 2.0.
+        if (char* dot = strchr(buf, '.')) {
+            if (number_of_decimals == 0)
+                *dot = '\0';
+            else if ((dot - buf) + 1 + number_of_decimals < (int)sizeof(buf))
+                dot[1 + number_of_decimals] = '\0';
+        }
+        printf("%s\n", buf);
+        d += step;
+    }
+}