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Add tables.mgetOrPutLazy #52

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11 changes: 11 additions & 0 deletions stew/shims/tables.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -9,4 +9,15 @@ template init*[A, B](T: type OrderedTableRef[A, B]): auto = newOrderedTable[A, B
template init*[A](T: type CountTable[A]): auto = initCountTable[A]()
template init*[A](T: type CountTableRef[A]): auto = newCountTable[A]()

template mgetOrPutLazy*[A, B](t: Table[A, B], key: A, val: B): var B =
type R = B

proc setter(loc: var R): var R =
if loc == default(R):

@arnetheduck arnetheduck Aug 8, 2020 •
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so if the collection had a value that was all zeroes (as opposed to not having a value), this will update it to val which doesn't seem to be the intent

@zah zah Aug 8, 2020 •
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Yes, this version of code comes with a number of caveats, but the idea of having a shim is that it allows us to capture the intention of the user until a more general and efficient version of the API is added to the tables module (as a template similar to withValue).

At the moment it's reasonable to use mostly with tables with ref values that don't need to store nil. The default function for the type should be cheap and not a valid value in the usage context.

@arnetheduck arnetheduck Aug 8, 2020

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well, the that's the point a bit - my intent is not to set if the value is the default - it's to set / create only if it's not in the collection already

loc = val
loc

setter(mgetOrPut(t, key, default(R)))

@arnetheduck arnetheduck Aug 7, 2020

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this looks pretty odd - ie if B is an array[10000, uint64], this will.. zero-init such an array, zeroinit another such array (default(R)) then compare all those values?


export tables, objects

5 changes: 3 additions & 2 deletions tests/all_tests.nim
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Expand Up @@ -15,10 +15,11 @@ import
test_bitops2,
test_bitseqs,
test_byteutils,
test_ctops,
test_endians2,
test_macros,
test_objects,
test_ptrops,
test_results,
test_varints,
test_ctops
test_tables,
test_varints
15 changes: 15 additions & 0 deletions tests/test_tables.nim
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@@ -0,0 +1,15 @@
import
unittest,
../stew/shims/tables

suite "tables":
test "mgetOrPutLazy":
var t = {"a": 10, "b": 20}.toTable
check t.mgetOrPutLazy("a", 30) == 10
check t.mgetOrPutLazy("c", 40) == 40

check t.mgetOrPutLazy("c", 20) == 40

t.mgetOrPutLazy("c", 20) = 15
check t.mgetOrPutLazy("c", 40) == 15