Add Filament_Extruder_map (#160)

* Optimize: extruder_filament_map

* Fix: {print_time}

FILAMENT_EXTRUDER_ANALYSIS_FINAL.md

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+# Filament-Extruder Mapping Analysis - Final Findings
+
+## The Key Discovery
+
+After investigating the original implementation vs the optimized version, I found the **critical missing piece**: `initialize_filament_extruder_map()`.
+
+## How It Actually Works
+
+### The Initialization Process
+
+**Called in PrintApply.cpp during apply():**
+```cpp
+// PrintApply.cpp line 1276
+this->initialize_filament_extruder_map();
+```
+
+**Implementation in Print.cpp (lines 497-544):**
+```cpp
+void Print::initialize_filament_extruder_map()
+{
+    m_filament_extruder_map.clear();
+
+    // Get the number of physical extruders
+    size_t physical_extruder_count = m_config.nozzle_diameter.values.size();
+
+    // Get ALL configured filaments (not just used ones)
+    std::vector<unsigned int> filament_extruders = this->extruders();
+    if (filament_extruders.empty()) {
+        size_t filament_count = m_config.filament_diameter.size();
+        for (size_t i = 0; i < filament_count; ++i) {
+            filament_extruders.push_back((unsigned int)i);
+        }
+    }
+
+    // Create mapping: filament_id -> physical_extruder_id
+    // Mapping formula: physical_extruder = filament_id % physical_extruder_count
+    for (unsigned int filament_idx : filament_extruders) {
+        int physical_extruder = filament_idx % physical_extruder_count;
+        m_filament_extruder_map[filament_idx] = physical_extruder;
+    }
+}
+```
+
+### The Result
+
+After initialization:
+- Filament 0 → Extruder 0
+- Filament 1 → Extruder 1
+- Filament 2 → Extruder 2
+- Filament 3 → Extruder 3
+- Filament 4 → Extruder 0 (modulo)
+- Filament 5 → Extruder 1 (modulo)
+- Filament 6 → Extruder 2 (modulo)
+- Filament 7 → Extruder 3 (modulo)
+
+**The map is ALWAYS populated before slicing!**
+
+## Why Both Versions Work
+
+### Original Implementation (9d423d0714)
+
+**get_physical_extruder():**
+```cpp
+int get_physical_extruder(int filament_idx) const {
+    auto it = m_filament_extruder_map.find(filament_idx);
+    // Fallback: identity mapping (filament 5 → extruder 5)
+    int physical_extruder_id = (it != m_filament_extruder_map.end()) ? it->second : filament_idx;
+    return physical_extruder_id;
+}
+```
+
+**Why it worked:** The fallback logic was **never reached** because `initialize_filament_extruder_map()` always populates the map with modulo mapping.
+
+### Optimized Implementation (989a53e124)
+
+**get_physical_extruder():**
+```cpp
+int get_physical_extruder(int filament_idx) const {
+    auto it = m_filament_extruder_map.find(filament_idx);
+    if (it != m_filament_extruder_map.end()) {
+        return it->second;
+    } else {
+        // Fallback: modulo mapping (filament 5 → extruder 1)
+        size_t physical_count = m_config.nozzle_diameter.values.size();
+        return filament_idx % physical_count;
+    }
+}
+```
+
+**Why it also works:** Same reason - the map is always populated. The fallback is just defensive programming.
+
+## What Actually Changed
+
+The key difference between the two versions is **NOT** the behavior during normal operation (both work the same), but:
+
+1. **Defensive programming:** The optimized version has a safer fallback (modulo instead of identity)
+2. **Code simplification:** The optimized version removed some redundant physical_extruder calculations in GCode.cpp
+3. **Better logging:** The optimized version has more detailed logging
+4. **Validation:** The optimized version added bounds checking in PrintApply.cpp
+
+## The Real Issue: Why 3MF Slicing Might Fail
+
+If the user is experiencing issues with >4 filament 3MF slicing, it's **NOT** because of the identity vs modulo fallback difference (since that code path is never reached).
+
+Possible causes:
+
+1. **initialize_filament_extruder_map() not being called:** Check if there's a code path that bypasses Print::apply()
+
+2. **Config array access using filament_id instead of physical_extruder_id:** The optimized version removed some explicit physical_extruder calculations in GCode.cpp. If those changes introduced direct filament_id access to config arrays, that would cause crashes.
+
+3. **3MF file format issues:** The 3MF file might have inconsistent filament/extruder configurations
+
+4. **Placeholder replacement:** GCode placeholder macros might still be using filament_id instead of physical_extruder_id
+
+## Code Changes That Matter
+
+### Potentially Problematic Change in GCode.cpp
+
+**Optimized version removed explicit physical_extruder calculation:**
+```cpp
+// BEFORE (9d423d0714):
+int previous_physical_extruder = (previous_extruder_id >= 0) ?
+    gcode_writer.get_physical_extruder(previous_extruder_id) : -1;
+int new_physical_extruder = gcode_writer.get_physical_extruder(new_extruder_id);
+float old_retract_length = (gcode_writer.extruder() != nullptr && previous_physical_extruder >= 0) ?
+    full_config.retraction_length.get_at(previous_physical_extruder) : 0;
+
+// AFTER (989a53e124):
+float old_retract_length = (gcode_writer.extruder() != nullptr && previous_extruder_id >= 0) ?
+    full_config.retraction_length.get_at(previous_extruder_id) : 0;  // Uses filament_id directly!
+```
+
+**This is a BUG!** The optimized version uses `previous_extruder_id` (filament_id) directly to access `retraction_length` array. This will cause array out-of-bounds access when filament_id >= physical_extruder_count.
+
+## Recommendation
+
+The optimized version (989a53e124) may have introduced regressions by removing explicit physical_extruder calculations. Need to:
+
+1. **Audit all config array access** in GCode.cpp to ensure physical_extruder_id is used
+2. **Add bounds checking** or use the PHYSICAL_EXTRUDER_CONFIG macro consistently
+3. **Test with 8-filament 3MF** on 4-extruder configuration
+
+## Summary
+
+| Aspect | Original (9d423d0714) | Optimized (989a53e124) |
+|--------|----------------------|------------------------|
+| **Modulo mapping** | ✓ Via initialize_filament_extruder_map() | ✓ Via initialize_filament_extruder_map() |
+| **Fallback logic** | Identity (never used) | Modulo (never used) |
+| **Config array access** | Explicit physical_extruder calculation | Some direct filament_id access (BUG?) |
+| **Safety** | Good | Potentially introduced bugs |
+
+**Bottom line:** Both versions use the same modulo mapping via `initialize_filament_extruder_map()`, but the optimized version may have introduced bugs by simplifying config array access.