Fix command registration and add command infra

src/commands/ping.c

@@ -0,0 +1,30 @@
+#include <concord/discord.h>
+#include <inttypes.h>
+#include <stdio.h>
+
+#include <hammy/command.h>
+#include <hammy/job.h>
+
+// Discord epoch, for turning a snowflake back into a wall-clock time.
+#define HAMMY_DISCORD_EPOCH_MS 1420070400000LL
+ 
+static int64_t hammy_snowflake_to_ms(u64snowflake id) {
+    return (int64_t)(id >> 22) + HAMMY_DISCORD_EPOCH_MS;
+}
+ 
+// Instant command: runs on the gateway thread, sends a fresh response.
+void hammy_cmd_ping(const hammy_job_t* job, struct discord* client) {
+    // Gateway heartbeat round trip, as measured by Concord.
+    int gatewayMs = discord_get_ping(client);
+ 
+    // Time from Discord minting the interaction to us handling it. This is the
+    // number that degrades under load, so it is the one worth showing.
+    int64_t handledMs = (int64_t)discord_timestamp(client) - hammy_snowflake_to_ms(job->id);
+ 
+    char body[256];
+    snprintf(body, sizeof(body),
+             "Pong!\nGateway: `%d ms`\nHandled in: `%" PRId64 " ms`",
+             gatewayMs, handledMs);
+ 
+    hammy_job_respond(job, client, body);
+}