sql/util.ts: Add a simple SQL record mapper

sql-bricks can write queries for us, and the mapper handles type
conversions to and from SQL-friendly strings.

src/sql/util.ts

@@ -0,0 +1,86 @@
+import snakeCase from "snake-case";
+
+import { Row } from "./api";
+
+interface ColMapper<F> {
+  _read(str: string): F;
+
+  _write(val: F): string;
+}
+
+type Spec<R> = {
+  [K in keyof R]: ColMapper<R[K]>;
+};
+
+/**
+ * Function objects describing the precise way in which our SQL driver
+ * transmits and receives values to the database as strings. Note that we could
+ * potentially have more than one SQL driver so this probably needs to be
+ * pushed down into `/sql/api.ts` somehow...
+ */
+export const T = {
+  bigint: {
+    _read: (str: string) => BigInt(str),
+    _write: (val: bigint) => val.toString(),
+  },
+  boolean: {
+    _read: (str: string) => str === "true",
+    _write: (val: boolean) => val.toString(),
+  },
+  number: {
+    _read: (str: string) => parseInt(str),
+    _write: (val: number) => val.toString(),
+  },
+  string: {
+    _read: (str: string) => str,
+    _write: (val: string) => val,
+  },
+  Date: {
+    _read: (str: string) => new Date(str),
+    _write: (val: Date) => val.toISOString(),
+  },
+};
+
+/**
+ * Boilerplate-reducing SQL <-> JavaScript record mapper. The TypeScript
+ * compiler will ensure that the mapping is correct.
+ */
+export function createSqlMapper<R>(spec: Spec<R>) {
+  const snaked = new Map<string, string>();
+  const colNames = new Array<string>();
+
+  for (const k in spec) {
+    const sk = snakeCase(k);
+
+    snaked.set(k, sk);
+    colNames.push(sk);
+  }
+
+  function readRow(row: Row): R {
+    const result = {} as R;
+
+    for (const k in spec) {
+      const sk = snaked.get(k);
+      const v = spec[k]._read(row[sk!]);
+
+      result[k] = v;
+    }
+
+    return result;
+  }
+
+  function writeRow(obj: R): Row {
+    const result = {};
+
+    for (const k in spec) {
+      const sk = snaked.get(k);
+      const v = spec[k]._write(obj[k]);
+
+      result[sk!] = v;
+    }
+
+    return result;
+  }
+
+  return { readRow, writeRow, colNames };
+}