Improve usage instructions

Author: mnvr

21.swift

@@ -294,7 +294,7 @@ var (grid, quantumGrid) = readInput()
 // let p1 = grid.reachable
 // print(p1)
 
-quantumMove(grid: &quantumGrid, steps: 10)
+quantumMove(grid: &quantumGrid, steps: 6)
 let p2 = quantumGrid.reachable
 print(quantumGrid)
 print(p2)

README.md

@@ -1,32 +1,53 @@
-Haskell¹ solutions to [Advent of Code 2023](https://adventofcode.com/2023).
+Haskell and Swift¹ solutions to [Advent of Code 2023](https://adventofcode.com/2023).
+
+Each solution is in a self-contained file with zero dependencies that can be
+run directly by the stock Haskell and Swift compilers. For example:
+
+    cat examples/18 | runghc 18.hs
+    cat examples/18 | swift 18.swift
+
+More details about running, including some convenient `make` invocations, are
+described below.
 
 <small>¹ The primary solutions are is Haskell. But for some problems, I've also
-done variations using other means and languages. </small>
+done variations using other means and languages, mostly Swift.</small>
 
 ## Running
 
-Install [Haskell using GHCup](https://www.haskell.org/ghcup/). Then, to run the
-latest solution on the latest of its example inputs, use
+Install [Haskell using GHCup](https://www.haskell.org/ghcup/), or
+[Swift](https://swift.org), depending on which solutions you want to run.
+
+Then, to run the latest solution on the latest of its example inputs, use
 
     make
 
 Other commands:
 
 -   `make test` – run on the actual input, and compare against the expected
-    answers.
--   `make o` - same as `make test`, but first compile the .hs file using GHC
-    with optimizations enabled. There's also a `make o2` variant of this.
+    answers. The actual inputs should be in `input/xx`, and the expected answers
+    in `answers/xx-a` and `answers/xx-b`.
+-   `make o` - same as `make test`, but first compile the file using `ghc` or
+    `swiftc` with optimizations enabled. There's also a `make o2` variant of
+    this for Haskell that uses the `-O2` level.
 -   `make verify` - Run `make test`, but for all days so far, in reverse. Print
     nice and pretty stats about all the days too.
 -   `make clean` - Clean temporary files (written to `out/`).
 
 To run the command on a specific day's program, just `touch` it so that it has
-the latest modification time - subsequent make invocations will pick it.
+the latest modification time - subsequent make invocations will pick it. So for
+example, if I wanted to run day 18, I can
 
-All of these are wrappers around the basic pattern of
+    touch 18.swift # or touch 18.hs, depending on which I wish to run
+    make
+
+All of make invocations are wrappers around the basic pattern of
 
     cat somefile | runghc xx.hs
 
+and
+
+    cat somefile | swift xx.swift
+
 Some of the solutions are done in shell scripts. To run these, the filename
 needs to be passed to the script instead of passing the input via stdin: