DVX_GUI/dvx/widgets/widgetCore.c

// widgetCore.c — Core widget infrastructure (alloc, tree ops, helpers)
//
// This file provides the foundation for the widget tree: allocation,
// parent-child linking, focus management, hit testing, and shared
// utility functions used across multiple widget types.
//
// Widgets form a tree using intrusive linked lists (firstChild/lastChild/
// nextSibling pointers inside each WidgetT). This is a singly-linked
// child list with a tail pointer for O(1) append. The tree is owned
// by its root, which is attached to a WindowT. Destroying the root
// recursively destroys all descendants.
//
// Memory allocation is plain malloc/free rather than an arena or pool.
// The widget count per window is typically small (tens to low hundreds),
// so the allocation overhead is negligible on target hardware. An arena
// approach was considered but rejected because widgets can be individually
// created and destroyed at runtime (dialog dynamics, tree item insertion),
// which doesn't map cleanly to an arena pattern.

#include "widgetInternal.h"

// ============================================================
// Global state for drag and popup tracking
// ============================================================
//
// These module-level pointers track ongoing UI interactions that span
// multiple mouse events (drags, popups, button presses). They are global
// rather than per-window because the DOS GUI is single-threaded and only
// one interaction can be active at a time.
//
// Each pointer is set when an interaction begins (e.g. mouse-down on a
// slider) and cleared when it ends (mouse-up). The event dispatcher in
// widgetEvent.c checks these before normal hit testing — active drags
// take priority over everything else.
//
// All of these must be NULLed when the pointed-to widget is destroyed,
// otherwise dangling pointers would cause crashes. widgetDestroyChildren()
// and wgtDestroy() handle this cleanup.

bool     sDebugLayout    = false;
WidgetT *sFocusedWidget  = NULL;   // currently focused widget (O(1) access, avoids tree walk)
WidgetT *sOpenPopup      = NULL;   // dropdown/combobox with open popup list
WidgetT *sPressedButton  = NULL;   // button being held down (tracks mouse in/out)
WidgetT *sDragSlider     = NULL;   // slider being dragged
WidgetT *sDrawingCanvas  = NULL;   // canvas receiving paint strokes
WidgetT *sDragTextSelect = NULL;   // text widget in drag-select mode
int32_t  sDragOffset     = 0;      // pixel offset from drag start to thumb center
WidgetT *sResizeListView = NULL;   // ListView undergoing column resize
int32_t  sResizeCol      = -1;     // which column is being resized
int32_t  sResizeStartX   = 0;      // mouse X at resize start
int32_t  sResizeOrigW    = 0;      // column width at resize start
bool     sResizeDragging = false;  // true once mouse moves during column resize
WidgetT *sDragSplitter      = NULL;   // splitter being dragged
int32_t  sDragSplitStart    = 0;      // mouse offset from splitter edge at drag start
WidgetT *sDragReorder       = NULL;   // list/tree widget in drag-reorder mode
WidgetT *sDragScrollbar     = NULL;   // widget whose scrollbar thumb is being dragged
int32_t  sDragScrollbarOff  = 0;      // mouse offset within thumb at drag start
int32_t  sDragScrollbarOrient = 0;    // 0=vertical, 1=horizontal


// ============================================================
// widgetAddChild
// ============================================================
//
// Appends a child to the end of the parent's child list. O(1)
// thanks to the lastChild tail pointer. The child list is singly-
// linked (nextSibling), which saves 4 bytes per widget vs doubly-
// linked and is sufficient because child removal is infrequent.

void widgetAddChild(WidgetT *parent, WidgetT *child) {
    child->parent      = parent;
    child->nextSibling = NULL;

    if (parent->lastChild) {
        parent->lastChild->nextSibling = child;
        parent->lastChild = child;
    } else {
        parent->firstChild = child;
        parent->lastChild  = child;
    }
}


// ============================================================
// widgetAlloc
// ============================================================
//
// Allocates and zero-initializes a new widget, links it to its
// class vtable via widgetClassTable[], and optionally adds it as
// a child of the given parent.
//
// The memset to 0 is intentional — it establishes sane defaults
// for all fields: NULL pointers, zero coordinates, no focus,
// no accel key, etc. Only visible and enabled default to true.
//
// The window pointer is inherited from the parent so that any
// widget in the tree can find its owning window without walking
// to the root.

WidgetT *widgetAlloc(WidgetT *parent, WidgetTypeE type) {
    WidgetT *w = (WidgetT *)malloc(sizeof(WidgetT));

    if (!w) {
        return NULL;
    }

    memset(w, 0, sizeof(*w));
    w->type    = type;
    w->wclass  = widgetClassTable[type];
    w->visible = true;
    w->enabled = true;

    if (parent) {
        w->window = parent->window;
        widgetAddChild(parent, w);
    }

    return w;
}


// ============================================================
// widgetClearFocus
// ============================================================

void widgetClearFocus(WidgetT *root) {
    if (!root) {
        return;
    }

    root->focused = false;

    for (WidgetT *c = root->firstChild; c; c = c->nextSibling) {
        widgetClearFocus(c);
    }
}


// ============================================================
// widgetCountVisibleChildren
// ============================================================

int32_t widgetCountVisibleChildren(const WidgetT *w) {
    int32_t count = 0;

    for (const WidgetT *c = w->firstChild; c; c = c->nextSibling) {
        if (c->visible) {
            count++;
        }
    }

    return count;
}


// ============================================================
// widgetDestroyChildren
// ============================================================
//
// Recursively destroys all descendants of a widget. Processes
// children depth-first (destroy grandchildren before the child
// itself) so that per-widget destroy callbacks see a consistent
// tree state.
//
// Critically, this function clears all global state pointers that
// reference destroyed widgets. Without this, any pending drag or
// focus state would become a dangling pointer. Each global is
// checked individually rather than cleared unconditionally to
// avoid disrupting unrelated ongoing interactions.

void widgetDestroyChildren(WidgetT *w) {
    WidgetT *child = w->firstChild;

    while (child) {
        WidgetT *next = child->nextSibling;
        widgetDestroyChildren(child);

        if (child->wclass && child->wclass->destroy) {
            child->wclass->destroy(child);
        }

        // Clear static references if they point to destroyed widgets
        if (sFocusedWidget == child) {
            sFocusedWidget = NULL;
        }

        if (sOpenPopup == child) {
            sOpenPopup = NULL;
        }

        if (sPressedButton == child) {
            sPressedButton = NULL;
        }

        if (sDragSlider == child) {
            sDragSlider = NULL;
        }

        if (sDrawingCanvas == child) {
            sDrawingCanvas = NULL;
        }

        if (sResizeListView == child) {
            sResizeListView = NULL;
            sResizeCol      = -1;
            sResizeDragging = false;
        }

        if (sDragScrollbar == child) {
            sDragScrollbar = NULL;
        }

        free(child);
        child = next;
    }

    w->firstChild = NULL;
    w->lastChild  = NULL;
}


// ============================================================
// widgetDropdownPopupRect
// ============================================================
//
// Calculates the screen rectangle for a dropdown/combobox popup list.
// Shared between Dropdown and ComboBox since they have identical
// popup positioning logic.
//
// The popup tries to open below the widget first. If there isn't
// enough room (popup would extend past the content area bottom),
// it flips to open above instead. This ensures the popup is always
// visible, even for dropdowns near the bottom of a window.
//
// Popup height is capped at DROPDOWN_MAX_VISIBLE items to prevent
// huge popups from dominating the screen.

void widgetDropdownPopupRect(WidgetT *w, const BitmapFontT *font, int32_t contentH, int32_t *popX, int32_t *popY, int32_t *popW, int32_t *popH) {
    int32_t itemCount = 0;

    if (w->type == WidgetDropdownE) {
        itemCount = w->as.dropdown.itemCount;
    } else if (w->type == WidgetComboBoxE) {
        itemCount = w->as.comboBox.itemCount;
    }

    int32_t visibleItems = itemCount;

    if (visibleItems > DROPDOWN_MAX_VISIBLE) {
        visibleItems = DROPDOWN_MAX_VISIBLE;
    }

    if (visibleItems < 1) {
        visibleItems = 1;
    }

    *popX = w->x;
    *popW = w->w;
    *popH = visibleItems * font->charHeight + 4; // 2px border each side

    // Try below first, then above if no room
    if (w->y + w->h + *popH <= contentH) {
        *popY = w->y + w->h;
    } else {
        *popY = w->y - *popH;

        if (*popY < 0) {
            *popY = 0;
        }
    }
}


// ============================================================
// widgetFindByAccel
// ============================================================
//
// Finds a widget with the given Alt+key accelerator. Recurses the
// tree depth-first, respecting visibility and enabled state.
//
// Special case for TabPage widgets: even if the tab page itself is
// not visible (inactive tab), its accelKey is still checked. This
// allows Alt+key to switch to a different tab. However, children
// of invisible tab pages are NOT searched — their accelerators
// should not be active when the tab is hidden.

WidgetT *widgetFindByAccel(WidgetT *root, char key) {
    if (!root || !root->enabled) {
        return NULL;
    }

    // Invisible tab pages: match the page itself (for tab switching)
    // but don't recurse into children (their accels shouldn't be active)
    if (!root->visible) {
        if (root->type == WidgetTabPageE && root->accelKey == key) {
            return root;
        }

        return NULL;
    }

    if (root->accelKey == key) {
        return root;
    }

    for (WidgetT *c = root->firstChild; c; c = c->nextSibling) {
        WidgetT *found = widgetFindByAccel(c, key);

        if (found) {
            return found;
        }
    }

    return NULL;
}


// ============================================================
// widgetFindNextFocusable
// ============================================================
//
// Implements Tab-order navigation: finds the next focusable widget
// after 'after' in depth-first tree order. The two-pass approach
// (search from 'after' to end, then wrap to start) ensures circular
// tabbing — Tab on the last focusable widget wraps to the first.
//
// The pastAfter flag tracks whether we've passed the 'after' widget
// during traversal. Once past it, the next focusable widget is the
// answer. This avoids collecting all focusable widgets into an array
// just to find the next one — the common case returns quickly.

static WidgetT *findNextFocusableImpl(WidgetT *w, WidgetT *after, bool *pastAfter) {
    if (!w->visible || !w->enabled) {
        return NULL;
    }

    if (after == NULL) {
        *pastAfter = true;
    }

    if (w == after) {
        *pastAfter = true;
    } else if (*pastAfter && widgetIsFocusable(w->type)) {
        return w;
    }

    for (WidgetT *c = w->firstChild; c; c = c->nextSibling) {
        WidgetT *found = findNextFocusableImpl(c, after, pastAfter);

        if (found) {
            return found;
        }
    }

    return NULL;
}

WidgetT *widgetFindNextFocusable(WidgetT *root, WidgetT *after) {
    bool pastAfter = false;
    WidgetT *found = findNextFocusableImpl(root, after, &pastAfter);

    if (found) {
        return found;
    }

    // Wrap around — search from the beginning
    pastAfter = true;
    return findNextFocusableImpl(root, NULL, &pastAfter);
}


// ============================================================
// widgetFindPrevFocusable
// ============================================================
//
// Shift+Tab navigation: finds the previous focusable widget.
// Unlike findNextFocusable which can short-circuit during traversal,
// finding the PREVIOUS widget requires knowing the full order.
// So this collects all focusable widgets into an array, finds the
// target's index, and returns index-1 (with wraparound).
//
// The explicit stack-based DFS (rather than recursion) is used here
// because we need to push children in reverse order to get the same
// left-to-right depth-first ordering as the recursive version.
// Fixed-size arrays (128 widgets, 64 stack depth) are adequate for
// any reasonable dialog layout and avoid dynamic allocation.

WidgetT *widgetFindPrevFocusable(WidgetT *root, WidgetT *before) {
    WidgetT *list[128];
    int32_t  count = 0;

    // Collect all focusable widgets via depth-first traversal
    WidgetT *stack[64];
    int32_t  top = 0;
    stack[top++] = root;

    while (top > 0) {
        WidgetT *w = stack[--top];

        if (!w->visible || !w->enabled) {
            continue;
        }

        if (widgetIsFocusable(w->type) && count < 128) {
            list[count++] = w;
        }

        // Push children in reverse order so first child is processed first
        WidgetT *children[64];
        int32_t  childCount = 0;

        for (WidgetT *c = w->firstChild; c; c = c->nextSibling) {
            if (childCount < 64) {
                children[childCount++] = c;
            }
        }

        for (int32_t i = childCount - 1; i >= 0; i--) {
            if (top < 64) {
                stack[top++] = children[i];
            }
        }
    }

    if (count == 0) {
        return NULL;
    }

    // Find 'before' in the list
    int32_t idx = -1;

    for (int32_t i = 0; i < count; i++) {
        if (list[i] == before) {
            idx = i;
            break;
        }
    }

    if (idx <= 0) {
        return list[count - 1]; // Wrap to last
    }

    return list[idx - 1];
}


// ============================================================
// widgetFrameBorderWidth
// ============================================================

int32_t widgetFrameBorderWidth(const WidgetT *w) {
    if (w->type != WidgetFrameE) {
        return 0;
    }

    if (w->as.frame.style == FrameFlatE) {
        return FRAME_FLAT_BORDER;
    }

    return FRAME_BEVEL_BORDER;
}


// ============================================================
// widgetHitTest
// ============================================================
//
// Recursive hit testing: finds the deepest (most specific) widget
// under the given coordinates. Returns the widget itself if no
// child is hit, or NULL if the point is outside this widget.
//
// Children are iterated front-to-back (first to last in the linked
// list), but the LAST match wins. This gives later siblings higher
// Z-order, which matches the painting order (later children paint
// on top of earlier ones). This is important for overlapping widgets,
// though in practice the layout engine rarely produces overlap.
//
// Widgets with WCLASS_NO_HIT_RECURSE stop the recursion — the parent
// widget handles all mouse events for its children. This is used by
// TreeView, ScrollPane, ListView, and Splitter, which need to manage
// their own internal regions (scrollbars, column headers, tree
// expand buttons) that don't correspond to child widgets.

WidgetT *widgetHitTest(WidgetT *w, int32_t x, int32_t y) {
    if (!w->visible) {
        return NULL;
    }

    if (x < w->x || x >= w->x + w->w || y < w->y || y >= w->y + w->h) {
        return NULL;
    }

    // Widgets with WCLASS_NO_HIT_RECURSE manage their own children
    if (w->wclass && (w->wclass->flags & WCLASS_NO_HIT_RECURSE)) {
        return w;
    }

    // Check children — take the last match (topmost in Z-order)
    WidgetT *hit = NULL;

    for (WidgetT *c = w->firstChild; c; c = c->nextSibling) {
        WidgetT *childHit = widgetHitTest(c, x, y);

        if (childHit) {
            hit = childHit;
        }
    }

    return hit ? hit : w;
}


// ============================================================
// widgetIsFocusable
// ============================================================

bool widgetIsFocusable(WidgetTypeE type) {
    return (widgetClassTable[type]->flags & WCLASS_FOCUSABLE) != 0;
}


// ============================================================
// widgetIsBoxContainer
// ============================================================
//
// Returns true for widget types that use the generic box layout.

bool widgetIsBoxContainer(WidgetTypeE type) {
    return (widgetClassTable[type]->flags & WCLASS_BOX_CONTAINER) != 0;
}


// ============================================================
// widgetIsHorizContainer
// ============================================================
//
// Returns true for container types that lay out children horizontally.

bool widgetIsHorizContainer(WidgetTypeE type) {
    return (widgetClassTable[type]->flags & WCLASS_HORIZ_CONTAINER) != 0;
}


// ============================================================
// widgetNavigateIndex
// ============================================================
//
// Shared keyboard navigation for list-like widgets (ListBox, Dropdown,
// ListView, etc.). Encapsulates the Up/Down/Home/End/PgUp/PgDn logic
// so each widget doesn't have to reimplement index clamping.
//
// Key values use the 0x100 flag to mark extended scan codes (arrow
// keys, Home, End, etc.) — this is the DVX convention for passing
// scan codes through the same int32_t channel as ASCII values.
//
// Returns -1 for unrecognized keys so callers can check whether the
// key was consumed.

int32_t widgetNavigateIndex(int32_t key, int32_t current, int32_t count, int32_t pageSize) {
    if (key == (0x50 | 0x100)) {
        // Down arrow
        if (current < count - 1) {
            return current + 1;
        }

        return current < 0 ? 0 : current;
    }

    if (key == (0x48 | 0x100)) {
        // Up arrow
        if (current > 0) {
            return current - 1;
        }

        return current < 0 ? 0 : current;
    }

    if (key == (0x47 | 0x100)) {
        // Home
        return 0;
    }

    if (key == (0x4F | 0x100)) {
        // End
        return count - 1;
    }

    if (key == (0x51 | 0x100)) {
        // Page Down
        int32_t n = current + pageSize;
        return n >= count ? count - 1 : n;
    }

    if (key == (0x49 | 0x100)) {
        // Page Up
        int32_t n = current - pageSize;
        return n < 0 ? 0 : n;
    }

    return -1;
}


// ============================================================
// widgetPaintPopupList
// ============================================================
//
// Shared popup list painting for Dropdown and ComboBox.

void widgetPaintPopupList(DisplayT *d, const BlitOpsT *ops, const BitmapFontT *font, const ColorSchemeT *colors, int32_t popX, int32_t popY, int32_t popW, int32_t popH, const char **items, int32_t itemCount, int32_t hoverIdx, int32_t scrollPos) {
    // Draw popup border
    BevelStyleT bevel;
    bevel.highlight = colors->windowHighlight;
    bevel.shadow    = colors->windowShadow;
    bevel.face      = colors->contentBg;
    bevel.width     = 2;
    drawBevel(d, ops, popX, popY, popW, popH, &bevel);

    // Draw items
    int32_t visibleItems = popH / font->charHeight;
    int32_t textX        = popX + TEXT_INPUT_PAD;
    int32_t textY        = popY + 2;
    int32_t textW        = popW - TEXT_INPUT_PAD * 2 - 4;

    for (int32_t i = 0; i < visibleItems && (scrollPos + i) < itemCount; i++) {
        int32_t  idx = scrollPos + i;
        int32_t  iy  = textY + i * font->charHeight;
        uint32_t ifg = colors->contentFg;
        uint32_t ibg = colors->contentBg;

        if (idx == hoverIdx) {
            ifg = colors->menuHighlightFg;
            ibg = colors->menuHighlightBg;
            rectFill(d, ops, popX + 2, iy, textW + TEXT_INPUT_PAD * 2, font->charHeight, ibg);
        }

        drawText(d, ops, font, textX, iy, items[idx], ifg, ibg, false);
    }
}


// ============================================================
// widgetScrollbarThumb
// ============================================================
//
// Calculates thumb position and size for a scrollbar track.
// Used by both the WM-level scrollbars and widget-internal scrollbars
// (ListBox, TreeView, etc.) to maintain consistent scrollbar behavior.
//
// The thumb size is proportional to visibleSize/totalSize — a larger
// visible area means a larger thumb, giving visual feedback about how
// much content is scrollable. SB_MIN_THUMB prevents the thumb from
// becoming too small to grab with a mouse.

void widgetScrollbarThumb(int32_t trackLen, int32_t totalSize, int32_t visibleSize, int32_t scrollPos, int32_t *thumbPos, int32_t *thumbSize) {
    *thumbSize = (trackLen * visibleSize) / totalSize;

    if (*thumbSize < SB_MIN_THUMB) {
        *thumbSize = SB_MIN_THUMB;
    }

    if (*thumbSize > trackLen) {
        *thumbSize = trackLen;
    }

    int32_t maxScroll = totalSize - visibleSize;

    if (maxScroll > 0) {
        *thumbPos = ((trackLen - *thumbSize) * scrollPos) / maxScroll;
    } else {
        *thumbPos = 0;
    }
}


// ============================================================
// widgetRemoveChild
// ============================================================
//
// Unlinks a child from its parent's child list. O(n) in the number
// of children because the singly-linked list requires walking to
// find the predecessor. This is acceptable because child removal
// is infrequent (widget destruction, tree item reordering).

void widgetRemoveChild(WidgetT *parent, WidgetT *child) {
    WidgetT *prev = NULL;

    for (WidgetT *c = parent->firstChild; c; c = c->nextSibling) {
        if (c == child) {
            if (prev) {
                prev->nextSibling = c->nextSibling;
            } else {
                parent->firstChild = c->nextSibling;
            }

            if (parent->lastChild == child) {
                parent->lastChild = prev;
            }

            child->nextSibling = NULL;
            child->parent      = NULL;
            return;
        }

        prev = c;
    }
}