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+# How to contribute a SQL function implementation?
+
+Steps
+1. Pick a `SQL functions` in [COMPAT.md](../../COMPAT.md) file with a No (not implemented yet) status.
+2. Create an issue for that function.
+3. Implement the function in a feature branch.
+4. Push it as a Merge Request, get it review.
+
+## An example with function `unixepoch(..)`
+
+> Note that the files, code location, steps might be not exactly the same because of refactor but the idea of the changes needed in each layer stays.
+
+[Issue #158](https://github.com/tursodatabase/limbo/issues/158) was created for it.
+Refer to commit [525f860](https://github.com/tursodatabase/limbo/commit/525f8600cacaff1dffc9e7fe9d274d89ed519149).
+
+```
+SQL_function --parser--> Func_enum ----> Instruction --VDBE--> Result
+```
+
+TODO for implementing the function:
+- analysis
+  - read and try out how the function works in SQLite.
+  - compare `explain` output of SQLite and Limbo.
+- add/ update the function definition in `functions.rs`.
+- add/ update how to function is translated from `definition` to `instruction` in virtual machine layer VDBE.
+- add/ update the function Rust execution code and tests in vdbe layer.
+- add/ update how the bytecode `Program` executes when steps into the function.
+- add/ update TCL tests for this function in limbo/testing.
+- update doc for function compatibility.
+
+### Analysis
+
+How `unixepoch` works in SQLite?
+```bash
+> sqlite3
+
+sqlite> explain select unixepoch('now');
+addr  opcode         p1    p2    p3    p4             p5  comment
+----  -------------  ----  ----  ----  -------------  --  -------------
+0     Init           0     6     0                    0   Start at 6
+1     Once           0     3     0                    0
+2     Function       0     0     2     unixepoch(-1)  0   r[2]=func()
+3     Copy           2     1     0                    0   r[1]=r[2]
+4     ResultRow      1     1     0                    0   output=r[1]
+5     Halt           0     0     0                    0
+6     Goto           0     1     0                    0
+```
+
+Comparing that with `Limbo`:
+```bash
+# created a sqlite database file
+> cargo run database.db
+
+Limbo v0.0.2
+Enter ".help" for usage hints.
+limbo> explain select unixtimestamp('now');
+Parse error: unknown function unixtimestamp
+```
+
+We can see that the function is not implemented yet so the Parser did not understand it and throw an error `Parse error: unknown function unixtimestamp`.
+- we only need to pay attention to opcode `Function` at addr 2. The rest is already set up in limbo.
+- we have up to 5 registers p1 to p5 for each opcode.
+
+### Function definition
+
+For limbo to understand the meaning of `unixtimestamp`, we need to define it as a Function somewhere.
+That place can be found currently in `core/functions.rs`. We need to edit 3 places
+1. add to ScalarFunc as `unixtimestamp` is a scalar function.
+2. add to Display to show the function as string in our program.
+3. add to `fn resolve_function(..)` of `impl Func` to enable parsing from str to this function.
+
+https://github.com/tursodatabase/limbo/blob/69e3dd28f77e59927da4313e517b2b428ede480d/core/function.rs#L86
+
+https://github.com/tursodatabase/limbo/blob/69e3dd28f77e59927da4313e517b2b428ede480d/core/function.rs#L131
+
+https://github.com/tursodatabase/limbo/blob/69e3dd28f77e59927da4313e517b2b428ede480d/core/function.rs#L331
+
+### Function translation
+
+How to translate the function into bytecode `Instruction`?
+
+https://github.com/tursodatabase/limbo/blob/525f8600cacaff1dffc9e7fe9d274d89ed519149/core/translate/expr.rs#L971C1-L989C48
+
+### Function execution
+
+https://github.com/tursodatabase/limbo/commit/525f8600cacaff1dffc9e7fe9d274d89ed519149#diff-839435241d4ffb648ad2d162bc6ba6a94f052309865251dc2aff36eaa14fa3c5L94-R111
+
+### Program bytecode execution
+
+https://github.com/tursodatabase/limbo/commit/525f8600cacaff1dffc9e7fe9d274d89ed519149#diff-14ede55920ec82e719d3d39a4c38a6b5c0d3e4fa1e7ff4d75e7f436820920fa7L33-R1392
+
+If there is no `time value` (no start register) , we want to execute the function with default param `'now'` as in [SQLite spec](https://www.sqlite.org/lang_datefunc.html#time_values).
+
+>  In all functions other than timediff(), the time-value (and all modifiers) may be omitted, in which case a time value of 'now' is assumed.
+
+```rust 
+if *start_reg == 0 {
+    let unixepoch: String =
+        exec_unixepoch(&OwnedValue::Text(Rc::new("now".to_string())))?;
+    state.registers[*dest] = OwnedValue::Text(Rc::new(unixepoch));
+}
+```
+
+### Adding tests
+
+Tests for `unixepoch` functions can be referenced from SQLite source code which is already very comprehensive. 
+- https://github.com/sqlite/sqlite/blob/f2b21a5f57e1a1db1a286c42af40563077635c3d/test/date3.test#L36
+- https://github.com/sqlite/sqlite/blob/f2b21a5f57e1a1db1a286c42af40563077635c3d/test/date.test#L604
+
+https://github.com/tursodatabase/limbo/commit/525f8600cacaff1dffc9e7fe9d274d89ed519149#diff-a262766efd02e804b8dc2ac5642f2061fb59a9388e437e9f000ff289110c9ec0L123-R145
+
+### Updating doc
+
+Update the COMPAT.md file to mark this function as implemented. Change Status to 
+- `Yes` if it is fully supported, 
+- `Partial` if supported but not fully yet compared to SQLite.
+
+![functions_compat_change.png](functions_compat_change.png)
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