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scratch-vm/test/snapshot/README.md

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+# Snapshot tests
+
+Snapshot testing runs the compiler on many of the test projects in test/fixtures/execute and verifies that the compiled output is identical to a pre-calculated "snapshot". This verifies:
+
+ - The compiler is deterministic (mostly)
+ - The compiler's output does not change unexpectedly
+ - Optimizations and type analysis in the generated code are working properly
+
+These snapshots are automatically verified as part of the integration tests. There is also a more readable CLI for humans. To verify the snapshots:
+
+```
+node test/snapshot
+```
+
+When the compiler's output or the test projects have intentionally changed, run:
+
+```
+node test/snapshot --update
+```
+
+Verify that the snapshot diff is what you expect, then commit the changes.