feature: Proper argument parsing

Use argp to parse command-line options. Add an "all" season to print
all dates for a year conveniently.

src/meeus-season.c

@@ -1,41 +1,113 @@
+#include <argp.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include "date.h"
 #include "meeus.h"
 
-enum season { SPRING, SUMMER, AUTUMN, WINTER };
+const char *argp_program_version = "0.1";
+const char *argp_program_bug_address = "<unused@domain>";
+static char args_doc[] = "SEASON YEAR";
+static char doc[] = "Calculate the date and time of solstices and equinoctes like Jean Meeus.";
 
-int parse_arguments(int argc, char **argv, int *year, enum season *season)
+struct arguments
 {
-    if(argc != 3 || strlen(argv[1]) > 7) return 1;
-
-    *year = atoi(argv[2]);
-         if(strcmp(argv[1], "spring") == 0){ *season = SPRING; }
-    else if(strcmp(argv[1], "summer") == 0){ *season = SUMMER; }
-    else if(strcmp(argv[1], "autumn") == 0){ *season = AUTUMN; }
-    else if(strcmp(argv[1], "winter") == 0){ *season = WINTER; }
-    else{ return 1; }
+    char *args[2];
+};
+
+/* Parse a single option. */
+static error_t parse_opt(int key, char *arg, struct argp_state *state)
+{
+    struct arguments *arguments = state->input;
+    switch (key){
+        case ARGP_KEY_ARG:
+            if(state->arg_num >= 2)
+                argp_usage(state);
+            arguments->args[state->arg_num] = arg;
+            break;
+        case ARGP_KEY_END:
+            if(state->arg_num < 2)
+                argp_usage(state);
+            break;
+        default:
+            return ARGP_ERR_UNKNOWN;
+    }
     return 0;
 }
 
-int main(int argc, char **argv)
+static struct argp_option options[] = {
+    { 0 }
+};
+static struct argp argp = { options, parse_opt, args_doc, doc, 0, 0, 0 };
+
+enum season {
+    SPRING,
+    SUMMER,
+    AUTUMN,
+    WINTER
+};
+
+static const enum season SEASONS[] = {
+    SPRING,
+    SUMMER,
+    AUTUMN,
+    WINTER
+};
+
+static const int YEAR_LIMIT = 20000;
+static const char *SEASON_ALL = "all";
+static const char *SEASON_NAMES[] = {
+    [SPRING] = "spring",
+    [SUMMER] = "summer",
+    [AUTUMN] = "autumn",
+    [WINTER] = "winter",
+};
+
+static int parse_command(struct arguments *a, int *year, const enum season **season, int *count)
 {
-    struct datetime dt;
-    int year;
-    enum season season;
+         if(strcmp(a->args[0], SEASON_ALL)       == 0){ *season = SEASONS;     *count = 4; }
+    else if(strcmp(a->args[0], SEASON_NAMES[0])  == 0){ *season = &SEASONS[0]; *count = 1; }
+    else if(strcmp(a->args[0], SEASON_NAMES[1])  == 0){ *season = &SEASONS[1]; *count = 1; }
+    else if(strcmp(a->args[0], SEASON_NAMES[2])  == 0){ *season = &SEASONS[2]; *count = 1; }
+    else if(strcmp(a->args[0], SEASON_NAMES[3])  == 0){ *season = &SEASONS[3]; *count = 1; }
+    else {
+        return 1;
+    }
 
-    if(parse_arguments(argc, argv, &year, &season))
-        return EXIT_FAILURE;
+    *year = atoi(a->args[1]);
+    if(*year > YEAR_LIMIT || *year < -YEAR_LIMIT) {
+        return 1;
+    }
+    return 0;
+}
 
+void print_season_datetime(enum season season, int year)
+{
+    struct datetime dt;
     switch(season){
     case SPRING: datetime_from_astronomical_jdn(&dt, spring_equinox(year)); break;
     case SUMMER: datetime_from_astronomical_jdn(&dt, summer_solstice(year)); break;
     case AUTUMN: datetime_from_astronomical_jdn(&dt, autumn_equinox(year)); break;
     case WINTER: datetime_from_astronomical_jdn(&dt, winter_solstice(year)); break;
     }
-
     datetime_print(&dt);
+}
+
+int main(int argc, char **argv)
+{
+    struct arguments arguments = { .args = { }, };
+    argp_parse(&argp, argc, argv, 0, 0, &arguments);
+
+    int year;
+    const enum season *seasons;
+    int count;
+    if(parse_command(&arguments, &year, &seasons, &count))
+        return EXIT_FAILURE;
+
+    for(int i=0; i<count; i++){
+        print_season_datetime(seasons[i], year);
+    }
+
     return EXIT_SUCCESS;
 }