Implement lists:flatten/1 as a NIF
#1954
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pguyot
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bettio
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src/libAtomVM/nifs.c
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| if (term_is_nonempty_list(t)) { | ||
| term t_head = term_get_list_head(t); | ||
| term t_tail = term_get_list_tail(t); | ||
| if (term_is_nonempty_list(t_tail)) { |
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It looks good, but my guess is that it can be replaced with a slightly optimized alternative implementation that doesn't push all values, but only when there are nested lists.
This should optimize the trivial case (such as [1, 2, 3, 4]), by avoiding copying to the temp stack the entire list.
Basically we should push t_tail to the stack every time t_head is a term_is_nonempty_list.
In that case t_tail is used as a "continuation value" as soon as the current list is finished.
e.g.
flatten([[1, 2], 5, 6, 7]):
iteration 0:
- t is [[1, 2], 5, 6, 7]
- t_head is [1, 2]
- t_tail is [5, 6, 7]
- t_tail is pushed to the stack
- t is set to [1, 2]
...
when t is []:
- t is popped from the stack
iteration n:
- t is [5, 6, 7]
...
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Good idea. I also optimized this further to reuse the longest reusable tail from original list.
NIF optimizes memory allocation by using a minimal stack to flatten, allocating on the heap only what is required to hold the flattened list and reusing the largest reusable tail from the original list. Signed-off-by: Paul Guyot <[email protected]>
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Nov 17, 2025
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See #1953
These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).
SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later