Add missing known-issues.md file referenced in README

known-issues.md

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+# Known Issues
+
+## Missing Output Files
+
+It has been observed that some generated files are missing.
+
+### Current Issue
+
+For unknown reasons, certain files are absent from the generated output. [This file](missing-files.txt) lists missing files.
+
+We are currently investigating the cause of this issue and exploring potential solutions.
+
+---
+
+## Changes in Element IDs or Names
+
+### Current Issue
+
+At times, certain elements within the electrical network experience changes in their names or may be replaced.
+
+Our policy regarding these changes is as follows:
+* If an element's name or ID changes, we aim to replace the new identifier with the old one to avoid impacting neural network training.
+* However, if an element is replaced due to damage or malfunction and the new element does not have identical characteristics (e.g., maximum intensity or voltage acceptance), it is treated as a distinct element.
+
+In practice, when element IDs change, the detection of the new name is not always performed correctly. This often leads to inconsistencies.
+
+### Solution Approach
+
+The detection mechanism is being improved to consider additional fields for better tracking of changes.
+
+1. For each element, all relevant identifying fields are concatenated using a "`__`" (double underscore) separator to form a unique `persistent_id`. This `persistent_id` remains consistent for a given element across different files.
+2. When comparing two files, we:
+   - Identify `persistent_id` values that were present in the first file but missing in the second.
+   - Identify `persistent_id` values that appear in the second file but were absent in the first.
+3. To match elements from the old file to the new file, we assume that changes in ID, name, and `persistent_id` should be relatively minor. We use Levenshtein distance calculations to find the best possible match.
+4. Steps 2 and 3 are repeated for all files in the dataset.
+5. Once the matrix mapping changing elements is established, all new IDs are replaced with their corresponding old IDs.
+
+---
+
+### Example Scenario
+
+We illustrate this approach with a simple example.
+
+#### First File:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<iidm:network xmlns:iidm="http://www.powsybl.org/schema/iidm/1_12" id="snapshot_2021-01-02-1000" caseDate="2021-01-02T10:00:00.000+01:00">
+...
+    <iidm:voltageLevel id="ARGIAP6" nominalV="225.0">
+        <iidm:switch id="ARGIAP6_ARGIA   6TR631    SA.1" name="ARGIA   6TR631    SA.1" />
+        <iidm:switch id="ARGIAP6_ARGIA   6AT761    SA.1" name="ARGIA   6AT761    SA.1" />
+    </iidm:voltageLevel>    
+...
+</iidm:network>
+```
+
+#### Second File:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<iidm:network xmlns:iidm="http://www.powsybl.org/schema/iidm/1_12" id="snapshot_2021-01-02-1100" caseDate="2021-01-02T11:00:00.000+01:00">
+...
+    <iidm:voltageLevel id="ARGIAP6" nominalV="225.0">
+        <iidm:switch id="ARGIAP6_ARGIA   6AT762    SA.1" name="ARGIA   6AT761    SA.1" />
+        <iidm:switch id="ARGIAP6_ARGIA   6TR631    SA.1" name="ARGIA   6TR632    SA.1" />
+    </iidm:voltageLevel>    
+...
+</iidm:network>
+```
+
+#### Observations:
+* The second file is more recent (see the `caseDate` attribute).
+* The first switch has a different ID in the second file.
+* The second switch has a different name in the second file.
+* The order of the two switches has changed, which can occur in real-world datasets.
+
+#### Persistent ID Computation
+
+For switches, the `persistent_id` is generated using:
+- Voltage level `id`
+- Switch `id`
+- Switch `name`
+- Switch nodes (`node1` and `node2`)
+
+##### Generated `persistent_id` values:
+**First File:**
+- `ARGIAP6__ARGIAP6_ARGIA 6TR631 SA.1__ARGIA 6TR631 SA.1__0__18` → `switch_1_a`
+- `ARGIAP6__ARGIAP6_ARGIA 6AT761 SA.1__ARGIA 6AT761 SA.1__0__6` → `switch_2_a`
+
+**Second File:**
+- `ARGIAP6__ARGIAP6_ARGIA 6AT762 SA.1__ARGIA 6AT761 SA.1__0__6` → `switch_1_b`
+- `ARGIAP6__ARGIAP6_ARGIA 6TR631 SA.1__ARGIA 6TR632 SA.1__0__18` → `switch_2_b`
+
+#### Levenshtein Distance Calculation:
+
+|            | Distance to switch_1_a | Distance to switch_1_b |
+|------------|----------------------|----------------------|
+| switch_2_a | 8                      | 2                      |
+| switch_2_b | 1                      | 11                     |
+
+Based on these distances, the algorithm correctly identifies:
+- `switch_1_a` corresponds to `switch_2_b`
+- `switch_1_b` corresponds to `switch_2_a`
+
+This aligns with the observed inversion of switch order in the second file.
+
+#### Final Adjustments
+
+After resolving ID mismatches, the second file is updated:
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<iidm:network xmlns:iidm="http://www.powsybl.org/schema/iidm/1_12" id="snapshot_2021-01-02-1100" caseDate="2021-01-02T11:00:00.000+01:00">
+...
+    <iidm:voltageLevel id="ARGIAP6" nominalV="225.0">
+        <iidm:switch id="ARGIAP6_ARGIA   6AT761    SA.1" name="ARGIA   6AT761    SA.1" />
+        <iidm:switch id="ARGIAP6_ARGIA   6TR631    SA.1" name="ARGIA   6TR631    SA.1" />
+    </iidm:voltageLevel>    
+...
+</iidm:network>
+```
+
+#### Conclusion
+While the switch order remains different, the correct identifiers have been restored, ensuring consistency across files.
+