automapper_validation/validator/

tree.rs

//! Merged AHB + EDIFACT tree for validation.
//!
//! [`build_validated_tree`] joins an [`AhbWorkflow`] (what should be there)
//! with an [`AssembledTree`] (what is there) into a single [`ValidatedTree`]
//! where each node carries both the AHB rule and the resolved EDIFACT value.

use std::collections::HashMap;

use crate::expr::ConditionExpr;

use super::validate::{AhbFieldRule, AhbWorkflow};
use mig_assembly::assembler::{
    AssembledGroup, AssembledGroupInstance, AssembledSegment, AssembledTree,
};

/// A single AHB field matched against EDIFACT data.
#[derive(Debug)]
pub struct AhbNode<'a> {
    /// The AHB field rule (segment path, ahb_status, codes, etc.)
    pub rule: &'a AhbFieldRule,
    /// The actual value found in the EDIFACT at this position. `None` if absent.
    pub value: Option<&'a str>,
    /// The full segment elements for cross-element access. `None` if segment absent.
    pub segment_elements: Option<&'a [Vec<String>]>,
}

/// A segment group instance matched against EDIFACT data.
#[derive(Debug)]
pub struct AhbGroupNode<'a> {
    /// Group ID (e.g., "SG4", "SG8").
    pub group_id: &'a str,
    /// AHB status of this group.
    pub ahb_status: Option<&'a str>,
    /// Fields in this group instance, resolved against EDIFACT segments.
    pub fields: Vec<AhbNode<'a>>,
    /// Child group instances.
    pub children: Vec<AhbGroupNode<'a>>,
}

/// AHB workflow merged with assembled EDIFACT data.
#[derive(Debug)]
pub struct ValidatedTree<'a> {
    /// The Pruefidentifikator.
    pub pruefidentifikator: &'a str,
    /// UB definitions for condition expansion.
    pub ub_definitions: &'a HashMap<String, ConditionExpr>,
    /// Root-level fields (outside segment groups).
    pub root_fields: Vec<AhbNode<'a>>,
    /// Top-level group instances.
    pub groups: Vec<AhbGroupNode<'a>>,
    /// Rules whose `mig_number` did not match any assembled group instance.
    ///
    /// These represent entirely-absent group variants (e.g., NAD+MS when only
    /// NAD+MR is present). `validate_tree` evaluates these using the flat
    /// `is_field_present`/`is_group_variant_absent` logic to correctly
    /// distinguish mandatory-but-missing variants from optional-and-absent ones.
    pub unmatched_rules: Vec<&'a AhbFieldRule>,
}

/// Build a [`ValidatedTree`] by merging an AHB workflow with an assembled EDIFACT tree.
///
/// The join key is `mig_number`: both [`AhbFieldRule::mig_number`] and
/// [`AssembledSegment::mig_number`] carry the MIG `Number` attribute that
/// uniquely identifies a segment variant.
pub fn build_validated_tree<'a>(
    workflow: &'a AhbWorkflow,
    tree: &'a AssembledTree,
) -> ValidatedTree<'a> {
    // Partition AHB rules into root-level and per-group buckets.
    let mut root_rules: Vec<&'a AhbFieldRule> = Vec::new();
    // Map from top-level group prefix (e.g., "SG2", "SG4") to rules.
    let mut group_rules: HashMap<String, Vec<&'a AhbFieldRule>> = HashMap::new();

    for rule in &workflow.fields {
        match extract_top_group(&rule.segment_path) {
            Some(group_id) => {
                group_rules
                    .entry(group_id.to_owned())
                    .or_default()
                    .push(rule);
            }
            None => {
                root_rules.push(rule);
            }
        }
    }

    // Resolve root-level fields against root segments.
    let root_fields = resolve_fields(&root_rules, &tree.segments);

    // Resolve groups recursively (depth 0 = top-level groups).
    let groups = resolve_groups(&tree.groups, &group_rules, 0);

    // Find rules whose mig_number wasn't matched to any tree node.
    // Collect all mig_numbers that appear in the assembled tree.
    let mut matched_mig_numbers: std::collections::HashSet<&str> =
        std::collections::HashSet::new();
    collect_matched_mig_numbers_from_groups(&groups, &mut matched_mig_numbers);
    // Also include root segment mig_numbers.
    for seg in &tree.segments {
        if let Some(ref num) = seg.mig_number {
            matched_mig_numbers.insert(num.as_str());
        }
    }

    let unmatched_rules: Vec<&AhbFieldRule> = workflow
        .fields
        .iter()
        .filter(|rule| {
            rule.mig_number
                .as_ref()
                .is_some_and(|num| !matched_mig_numbers.contains(num.as_str()))
        })
        .collect();

    ValidatedTree {
        pruefidentifikator: &workflow.pruefidentifikator,
        ub_definitions: &workflow.ub_definitions,
        root_fields,
        groups,
        unmatched_rules,
    }
}

/// Collect all mig_numbers that nodes in the tree were resolved against.
///
/// This walks the assembled tree structure (not the AHB nodes) to find
/// which mig_numbers are "claimed" by existing group instances.
fn collect_matched_mig_numbers_from_groups<'a>(
    groups: &[AhbGroupNode<'a>],
    out: &mut std::collections::HashSet<&'a str>,
) {
    for group in groups {
        for node in &group.fields {
            if let Some(ref num) = node.rule.mig_number {
                if node.value.is_some() || node.segment_elements.is_some() {
                    // Rule was matched to an actual segment.
                    out.insert(num.as_str());
                }
            }
            // Also count rules that are in the tree at all (even with None value)
            // because they were accepted by the variant filter.
            if let Some(ref num) = node.rule.mig_number {
                out.insert(num.as_str());
            }
        }
        collect_matched_mig_numbers_from_groups(&group.children, out);
    }
}

/// Extract the top-level segment group from a segment path.
///
/// `"SG4/DTM/C507/2380"` -> `Some("SG4")`
/// `"SG4/SG5/LOC/3225"` -> `Some("SG4")`
/// `"BGM/1004"` -> `None`
fn extract_top_group(segment_path: &str) -> Option<&str> {
    let first = segment_path.split('/').next()?;
    if first.starts_with("SG") {
        Some(first)
    } else {
        None
    }
}

/// Extract the child group prefix from a path that already had parent groups stripped.
///
/// `"SG5/LOC/3225"` -> `Some("SG5")`
/// `"DTM/C507/2380"` -> `None`
fn extract_child_group(stripped_path: &str) -> Option<&str> {
    let first = stripped_path.split('/').next()?;
    if first.starts_with("SG") {
        Some(first)
    } else {
        None
    }
}

/// Resolve AHB fields against a list of assembled segments.
///
/// For each rule, tries to find a matching segment by `mig_number` first,
/// then falls back to segment tag matching. Rules that don't match any
/// segment get `value: None` — condition evaluation decides if that's an error.
fn resolve_fields<'a>(
    rules: &[&'a AhbFieldRule],
    segments: &'a [AssembledSegment],
) -> Vec<AhbNode<'a>> {
    rules
        .iter()
        .map(|rule| {
            let matched_segment = find_segment(rule, segments);
            let (value, elements) = match matched_segment {
                Some(seg) => {
                    let val = extract_value(seg, rule);
                    (val, Some(seg.elements.as_slice()))
                }
                None => (None, None),
            };
            AhbNode {
                rule,
                value,
                segment_elements: elements,
            }
        })
        .collect()
}

/// Find the assembled segment matching an AHB field rule.
///
/// Matching strategy when the rule has a `mig_number`:
/// 1. **Strict match**: find a segment with the exact `mig_number`.
/// 2. **Tag fallback**: if no segment has that `mig_number`, fall back to tag
///    matching — but ONLY against segments that don't have a *different*
///    `mig_number` assigned. This prevents cross-matching between same-tag
///    segments with known identities (e.g., DTM+92 rules must not match a
///    DTM+93 segment that has `mig_number` "00024").
/// 3. Segments with `mig_number: None` (unattributed) are eligible for the
///    tag fallback, preserving backward compatibility with greedy assembly.
fn find_segment<'a>(
    rule: &AhbFieldRule,
    segments: &'a [AssembledSegment],
) -> Option<&'a AssembledSegment> {
    if let Some(ref mig_num) = rule.mig_number {
        // 1. Strict mig_number match.
        if let Some(seg) = segments
            .iter()
            .find(|s| s.mig_number.as_deref() == Some(mig_num.as_str()))
        {
            return Some(seg);
        }

        // 2. Tag fallback, excluding segments with a conflicting mig_number.
        let tag = extract_segment_tag(&rule.segment_path)?;
        return segments.iter().find(|s| {
            s.tag == tag && !s.mig_number.as_ref().is_some_and(|m| m.as_str() != mig_num.as_str())
        });
    }

    // No mig_number on rule — pure tag match.
    let tag = extract_segment_tag(&rule.segment_path)?;
    segments.iter().find(|s| s.tag == tag)
}

/// Extract the segment tag from a segment path.
///
/// `"SG4/DTM/C507/2380"` -> `"DTM"` (first non-SG component)
/// `"BGM/1004"` -> `"BGM"`
fn extract_segment_tag(segment_path: &str) -> Option<&str> {
    segment_path
        .split('/')
        .find(|part| !part.starts_with("SG"))
}

/// Extract a field value from an assembled segment using element/component indices.
fn extract_value<'a>(segment: &'a AssembledSegment, rule: &AhbFieldRule) -> Option<&'a str> {
    let elem_idx = rule.element_index.unwrap_or(0);
    let comp_idx = rule.component_index.unwrap_or(0);

    let element = segment.elements.get(elem_idx)?;
    let component = element.get(comp_idx)?;

    if component.is_empty() {
        None
    } else {
        Some(component.as_str())
    }
}

/// Resolve assembled groups against grouped AHB rules, recursively.
///
/// `depth` is the number of group prefixes to strip from each rule's
/// `segment_path` before classifying it as direct or child.
fn resolve_groups<'a>(
    assembled_groups: &'a [AssembledGroup],
    group_rules: &HashMap<String, Vec<&'a AhbFieldRule>>,
    depth: usize,
) -> Vec<AhbGroupNode<'a>> {
    let mut result = Vec::new();

    for assembled_group in assembled_groups {
        let rules = group_rules.get(&assembled_group.group_id);

        for instance in &assembled_group.repetitions {
            let node =
                resolve_group_instance(&assembled_group.group_id, instance, rules, depth);
            result.push(node);
        }
    }

    result
}

/// Strip `n` leading group prefixes from a segment path.
///
/// `strip_n_groups("SG4/SG5/LOC/3225", 2)` -> `"LOC/3225"`
fn strip_n_groups(path: &str, n: usize) -> &str {
    let mut rest = path;
    for _ in 0..n {
        match rest.find('/') {
            Some(idx) => rest = &rest[idx + 1..],
            None => return rest,
        }
    }
    rest
}

/// Resolve a single group instance.
///
/// `depth` is how many group prefixes have been consumed so far (0 for top-level groups).
///
/// Rules are filtered to this instance by `mig_number`: a rule only applies if
/// its `mig_number` matches a segment in this instance (or a segment in a child
/// group instance). This prevents rules for SG8/SEQ+Z79 from generating false
/// missing-field errors against an SG8/SEQ+Z01 rep.
fn resolve_group_instance<'a>(
    group_id: &'a str,
    instance: &'a AssembledGroupInstance,
    rules: Option<&Vec<&'a AhbFieldRule>>,
    depth: usize,
) -> AhbGroupNode<'a> {
    // We need to strip (depth + 1) group prefixes to get below this group level.
    let strip_count = depth + 1;

    // Use the variant's full set of mig_numbers (from MIG definition) to
    // determine which rules belong to this instance. This includes numbers
    // for segments that may be absent — so missing-field detection still works.
    // Falls back to collecting from present segments if variant_mig_numbers is empty.
    let variant_numbers: std::collections::HashSet<&str> = if !instance.variant_mig_numbers.is_empty() {
        instance.variant_mig_numbers.iter().map(|s| s.as_str()).collect()
    } else {
        collect_instance_mig_numbers(instance)
    };

    let mut direct_rules: Vec<&'a AhbFieldRule> = Vec::new();
    let mut child_group_rules: HashMap<String, Vec<&'a AhbFieldRule>> = HashMap::new();
    let mut ahb_status: Option<&'a str> = None;

    if let Some(rules) = rules {
        for rule in rules {
            // Skip rules whose mig_number doesn't belong to this variant.
            // Rules without mig_number pass through (tag-based fallback).
            if let Some(ref rule_mig) = rule.mig_number {
                if !variant_numbers.contains(rule_mig.as_str()) {
                    continue;
                }
            }

            // Strip all parent group prefixes plus this group to get the relative path.
            let stripped = strip_n_groups(&rule.segment_path, strip_count);

            if let Some(child_group_id) = extract_child_group(stripped) {
                child_group_rules
                    .entry(child_group_id.to_owned())
                    .or_default()
                    .push(rule);
            } else {
                direct_rules.push(rule);
            }

            // Capture the parent group AHB status if present.
            if ahb_status.is_none() {
                if let Some(ref status) = rule.parent_group_ahb_status {
                    ahb_status = Some(status.as_str());
                }
            }
        }
    }

    // Resolve direct fields against instance segments.
    let fields = resolve_fields(&direct_rules, &instance.segments);

    // Recurse into child groups one level deeper.
    let children = resolve_groups(&instance.child_groups, &child_group_rules, strip_count);

    AhbGroupNode {
        group_id,
        ahb_status,
        fields,
        children,
    }
}

/// Collect all mig_numbers present in a group instance, including child groups recursively.
fn collect_instance_mig_numbers(instance: &AssembledGroupInstance) -> std::collections::HashSet<&str> {
    let mut numbers = std::collections::HashSet::new();
    for seg in &instance.segments {
        if let Some(ref num) = seg.mig_number {
            numbers.insert(num.as_str());
        }
    }
    for child_group in &instance.child_groups {
        for child_instance in &child_group.repetitions {
            numbers.extend(collect_instance_mig_numbers(child_instance));
        }
    }
    numbers
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::validator::validate::{AhbFieldRule, AhbWorkflow};
    use mig_assembly::assembler::{
        AssembledGroup, AssembledGroupInstance, AssembledSegment, AssembledTree,
    };
    use std::collections::{BTreeMap, HashMap};

    fn empty_workflow() -> AhbWorkflow {
        AhbWorkflow {
            pruefidentifikator: "11001".to_string(),
            description: String::new(),
            communication_direction: None,
            fields: vec![],
            ub_definitions: HashMap::new(),
        }
    }

    fn empty_tree() -> AssembledTree {
        AssembledTree {
            segments: vec![],
            groups: vec![],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        }
    }

    fn make_segment(tag: &str, elements: Vec<Vec<&str>>, mig_number: Option<&str>) -> AssembledSegment {
        AssembledSegment {
            tag: tag.to_string(),
            elements: elements
                .into_iter()
                .map(|e| e.into_iter().map(|s| s.to_string()).collect())
                .collect(),
            mig_number: mig_number.map(|s| s.to_string()),
        }
    }

    fn make_rule(
        segment_path: &str,
        name: &str,
        ahb_status: &str,
        mig_number: Option<&str>,
        element_index: Option<usize>,
        component_index: Option<usize>,
    ) -> AhbFieldRule {
        AhbFieldRule {
            segment_path: segment_path.to_string(),
            name: name.to_string(),
            ahb_status: ahb_status.to_string(),
            codes: vec![],
            parent_group_ahb_status: None,
            element_index,
            component_index,
            mig_number: mig_number.map(|s| s.to_string()),
        }
    }

    #[test]
    fn test_empty_workflow_empty_tree() {
        let workflow = empty_workflow();
        let tree = empty_tree();
        let result = build_validated_tree(&workflow, &tree);

        assert_eq!(result.pruefidentifikator, "11001");
        assert!(result.root_fields.is_empty());
        assert!(result.groups.is_empty());
    }

    #[test]
    fn test_root_field_matches_root_segment() {
        let mut workflow = empty_workflow();
        workflow.fields.push(make_rule(
            "BGM/C002/1001",
            "Nachrichtentyp",
            "X",
            Some("0001"),
            Some(0),
            Some(0),
        ));

        let tree = AssembledTree {
            segments: vec![make_segment("BGM", vec![vec!["E01"]], Some("0001"))],
            groups: vec![],
            post_group_start: 1,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);

        assert_eq!(result.root_fields.len(), 1);
        let node = &result.root_fields[0];
        assert_eq!(node.value, Some("E01"));
        assert!(node.segment_elements.is_some());
        assert_eq!(node.rule.name, "Nachrichtentyp");
    }

    #[test]
    fn test_sg4_dtm_mig_number_matching() {
        // Two DTM segments with different mig_numbers.
        // AHB rule for DTM+92 (mig_number "0082") should match the correct one.
        let mut workflow = empty_workflow();

        // Rule for DTM+92: element 0 = qualifier "92", element 1 = the date value.
        workflow.fields.push(make_rule(
            "SG4/DTM/C507/2380",
            "Eingangsdatum",
            "X",
            Some("0082"),
            Some(1), // date value is element 1
            Some(0),
        ));

        // Rule for DTM+137.
        workflow.fields.push(make_rule(
            "SG4/DTM/C507/2380",
            "Dokumentendatum",
            "X",
            Some("0083"),
            Some(1),
            Some(0),
        ));

        let tree = AssembledTree {
            segments: vec![],
            groups: vec![AssembledGroup {
                group_id: "SG4".to_string(),
                repetitions: vec![AssembledGroupInstance {
                    segments: vec![
                        make_segment(
                            "DTM",
                            vec![vec!["92"], vec!["20260101"]],
                            Some("0082"),
                        ),
                        make_segment(
                            "DTM",
                            vec![vec!["137"], vec!["20260401"]],
                            Some("0083"),
                        ),
                    ],
                    child_groups: vec![],
                    entry_mig_number: None,
                    variant_mig_numbers: vec![],
                    skipped_segments: vec![],
                }],
            }],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);

        assert_eq!(result.groups.len(), 1);
        let sg4 = &result.groups[0];
        assert_eq!(sg4.group_id, "SG4");
        assert_eq!(sg4.fields.len(), 2);

        // Eingangsdatum (mig_number 0082) should get DTM+92's date.
        let eingangsdatum = &sg4.fields[0];
        assert_eq!(eingangsdatum.rule.name, "Eingangsdatum");
        assert_eq!(eingangsdatum.value, Some("20260101"));

        // Dokumentendatum (mig_number 0083) should get DTM+137's date.
        let dokumentendatum = &sg4.fields[1];
        assert_eq!(dokumentendatum.rule.name, "Dokumentendatum");
        assert_eq!(dokumentendatum.value, Some("20260401"));
    }

    #[test]
    fn test_rule_filtered_to_correct_variant() {
        // A rule with mig_number "0099" is filtered out from an instance
        // that doesn't contain any segment with that mig_number.
        // This prevents false missing-field errors for wrong SG8 variants.
        let mut workflow = empty_workflow();
        workflow.fields.push(make_rule(
            "SG4/RFF/C506/1154",
            "Referenz",
            "X",
            Some("0099"),
            Some(0),
            Some(1),
        ));

        let tree = AssembledTree {
            segments: vec![],
            groups: vec![AssembledGroup {
                group_id: "SG4".to_string(),
                repetitions: vec![AssembledGroupInstance {
                    segments: vec![], // No segments — wrong variant
                    child_groups: vec![],
                    entry_mig_number: None,
                    variant_mig_numbers: vec![],
                    skipped_segments: vec![],
                }],
            }],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);
        assert_eq!(result.groups.len(), 1);
        // Rule is filtered out — its mig_number doesn't match this instance
        assert_eq!(result.groups[0].fields.len(), 0);
    }

    #[test]
    fn test_missing_segment_within_correct_variant() {
        // A rule with mig_number "0099" IS included when the instance's
        // variant_mig_numbers lists it — even though the segment is absent.
        // This enables missing-field detection within the correct variant.
        let mut workflow = empty_workflow();
        // Entry segment rule (present)
        workflow.fields.push(make_rule(
            "SG4/SEQ/1229",
            "Qualifier",
            "X",
            Some("0098"),
            Some(0),
            Some(0),
        ));
        // Second segment rule (absent — should report missing)
        workflow.fields.push(make_rule(
            "SG4/RFF/C506/1154",
            "Referenz",
            "X",
            Some("0099"),
            Some(0),
            Some(1),
        ));

        let tree = AssembledTree {
            segments: vec![],
            groups: vec![AssembledGroup {
                group_id: "SG4".to_string(),
                repetitions: vec![AssembledGroupInstance {
                    segments: vec![
                        // Only the entry segment — RFF is missing
                        make_segment("SEQ", vec![vec!["Z01"]], Some("0098")),
                    ],
                    child_groups: vec![],
                    entry_mig_number: Some("0098".to_string()),
                    // variant_mig_numbers includes both "0098" (SEQ) and "0099" (RFF)
                    variant_mig_numbers: vec!["0098".to_string(), "0099".to_string()],
                    skipped_segments: vec![],
                }],
            }],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);
        assert_eq!(result.groups.len(), 1);
        // Both rules match because variant_mig_numbers includes both
        assert_eq!(result.groups[0].fields.len(), 2);
        assert_eq!(result.groups[0].fields[0].rule.name, "Qualifier");
        assert_eq!(result.groups[0].fields[0].value, Some("Z01"));
        // RFF is missing — value is None (condition eval will report it)
        assert_eq!(result.groups[0].fields[1].rule.name, "Referenz");
        assert_eq!(result.groups[0].fields[1].value, None);
    }

    #[test]
    fn test_missing_group_variant_populates_unmatched_rules() {
        // AHB requires two SG2 variants: NAD+MS (mig "0010") and NAD+MR (mig "0011").
        // EDIFACT only has NAD+MR. Rules for NAD+MS must appear in unmatched_rules
        // so validate_tree can report them as missing using flat logic.
        let mut workflow = empty_workflow();

        // NAD+MS qualifier rule
        workflow.fields.push(make_rule(
            "SG2/NAD/3035",
            "MP-ID Absender Qualifier",
            "X",
            Some("0010"),
            Some(0),
            Some(0),
        ));
        // NAD+MS ID rule
        workflow.fields.push(make_rule(
            "SG2/NAD/C082/3039",
            "MP-ID Absender",
            "X",
            Some("0010"),
            Some(1),
            Some(0),
        ));
        // NAD+MR qualifier rule
        workflow.fields.push(make_rule(
            "SG2/NAD/3035",
            "MP-ID Empfänger Qualifier",
            "X",
            Some("0011"),
            Some(0),
            Some(0),
        ));
        // NAD+MR ID rule
        workflow.fields.push(make_rule(
            "SG2/NAD/C082/3039",
            "MP-ID Empfänger",
            "X",
            Some("0011"),
            Some(1),
            Some(0),
        ));

        // Only NAD+MR present in assembled tree.
        let tree = AssembledTree {
            segments: vec![],
            groups: vec![AssembledGroup {
                group_id: "SG2".to_string(),
                repetitions: vec![AssembledGroupInstance {
                    segments: vec![make_segment(
                        "NAD",
                        vec![vec!["MR"], vec!["9900269000000", "", "293"]],
                        Some("0011"),
                    )],
                    child_groups: vec![],
                    entry_mig_number: Some("0011".to_string()),
                    variant_mig_numbers: vec!["0011".to_string()],
                    skipped_segments: vec![],
                }],
            }],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);

        // Tree should have 1 SG2 group node (NAD+MR).
        assert_eq!(result.groups.len(), 1);
        assert_eq!(result.groups[0].fields.len(), 2);
        assert_eq!(result.groups[0].fields[0].value, Some("MR"));

        // NAD+MS rules should be in unmatched_rules.
        assert_eq!(
            result.unmatched_rules.len(),
            2,
            "Expected 2 unmatched rules (NAD+MS), got {}",
            result.unmatched_rules.len()
        );
        assert_eq!(result.unmatched_rules[0].name, "MP-ID Absender Qualifier");
        assert_eq!(result.unmatched_rules[1].name, "MP-ID Absender");
    }

    #[test]
    fn test_entirely_absent_group_populates_unmatched_rules() {
        // AHB requires SG2 with NAD+MS (mig "0010"), but no SG2 exists at all.
        let mut workflow = empty_workflow();
        workflow.fields.push(make_rule(
            "SG2/NAD/3035",
            "MP-ID Absender Qualifier",
            "X",
            Some("0010"),
            Some(0),
            Some(0),
        ));

        let tree = empty_tree(); // No groups at all.

        let result = build_validated_tree(&workflow, &tree);

        // No tree nodes.
        assert!(result.groups.is_empty());

        // The rule should be in unmatched_rules.
        assert_eq!(
            result.unmatched_rules.len(),
            1,
            "Expected 1 unmatched rule, got {}",
            result.unmatched_rules.len()
        );
        assert_eq!(result.unmatched_rules[0].name, "MP-ID Absender Qualifier");
    }

    #[test]
    fn test_fallback_to_tag_when_no_mig_number() {
        let mut workflow = empty_workflow();
        workflow.fields.push(make_rule(
            "BGM/C002/1001",
            "Nachrichtentyp",
            "X",
            None, // No mig_number — should fall back to tag.
            Some(0),
            Some(0),
        ));

        let tree = AssembledTree {
            segments: vec![make_segment("BGM", vec![vec!["E01"]], None)],
            groups: vec![],
            post_group_start: 1,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);
        assert_eq!(result.root_fields.len(), 1);
        assert_eq!(result.root_fields[0].value, Some("E01"));
    }

    #[test]
    fn test_nested_child_groups() {
        let mut workflow = empty_workflow();
        // A rule in SG4/SG5.
        workflow.fields.push(make_rule(
            "SG4/SG5/LOC/C517/3225",
            "Marktlokations-ID",
            "X",
            Some("0050"),
            Some(0),
            Some(0),
        ));

        let tree = AssembledTree {
            segments: vec![],
            groups: vec![AssembledGroup {
                group_id: "SG4".to_string(),
                repetitions: vec![AssembledGroupInstance {
                    segments: vec![],
                    entry_mig_number: None,
                    child_groups: vec![AssembledGroup {
                        group_id: "SG5".to_string(),
                        repetitions: vec![AssembledGroupInstance {
                            segments: vec![make_segment(
                                "LOC",
                                vec![vec!["DE00012345678"]],
                                Some("0050"),
                            )],
                            child_groups: vec![],
                            entry_mig_number: None,
                            variant_mig_numbers: vec![],
                            skipped_segments: vec![],
                        }],
                    }],
                    variant_mig_numbers: vec![],
                    skipped_segments: vec![],
                }],
            }],
            post_group_start: 0,
            inter_group_segments: BTreeMap::new(),
        };

        let result = build_validated_tree(&workflow, &tree);
        assert_eq!(result.groups.len(), 1);
        let sg4 = &result.groups[0];
        assert_eq!(sg4.children.len(), 1);

        let sg5 = &sg4.children[0];
        assert_eq!(sg5.group_id, "SG5");
        assert_eq!(sg5.fields.len(), 1);
        assert_eq!(sg5.fields[0].value, Some("DE00012345678"));
        assert_eq!(sg5.fields[0].rule.name, "Marktlokations-ID");
    }
}