TerritoryBoundary.cpp

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/* Copyright (C) 2012 Wildfire Games.
 * This file is part of 0 A.D.
 *
 * 0 A.D. is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * 0 A.D. is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "precompiled.h"
#include "TerritoryBoundary.h"

#include <algorithm> // for reverse

#include "graphics/Terrain.h"
#include "simulation2/components/ICmpTerritoryManager.h"

std::vector<STerritoryBoundary> CTerritoryBoundaryCalculator::ComputeBoundaries(const Grid<u8>* territory)
{
    std::vector<STerritoryBoundary> boundaries;

    // Copy the territories grid so we can mess with it
    Grid<u8> grid(*territory);

    // Some constants for the border walk
    CVector2D edgeOffsets[] = {
        CVector2D(0.5f, 0.0f),
        CVector2D(1.0f, 0.5f),
        CVector2D(0.5f, 1.0f),
        CVector2D(0.0f, 0.5f)
    };

    // syntactic sugar
    const u8 TILE_BOTTOM = 0;
    const u8 TILE_RIGHT = 1;
    const u8 TILE_TOP = 2;
    const u8 TILE_LEFT = 3;

    const int CURVE_CW = -1;
    const int CURVE_CCW = 1;

    // === Find territory boundaries ===
    // 
    // The territory boundaries delineate areas of tiles that belong to the same player, and that all have the same 
    // connected-to-a-root-influence-entity status (see also STerritoryBoundary for a more wordy definition). Note that the grid 
    // values contain bit-packed information (i.e. not just the owning player ID), so we must be careful to only compare grid 
    // values using the player ID and connected flag bits. The joint mask to select these is referred to as the discriminator mask.
    // 
    // The idea is to scan the (i,j)-grid going up row by row and look for tiles that have a different territory assignment from 
    // the one right underneath it (or, if it's a tile on the first row, they need only have a territory assignment). These tiles 
    // are necessarily edge tiles of a territory, and hence a territory boundary must pass through their bottom edge. Therefore, 
    // we start tracing the outline of the territory starting from said bottom edge, and go CCW around the territory boundary.
    // Tracing continues until the starting point is reached, at which point the boundary is complete.
    // 
    // While tracing a boundary, every tile in which the boundary passes through the bottom edge are marked as 'processed', so that
    // we know not to start a new run from these tiles when scanning continues (when the boundary is complete). This information 
    // is maintained in the grid values themselves by means of the 'processed' bit mask (stressing the importance of using the 
    // discriminator mask to compare only player ID and connected flag).
    // 
    // Thus, we can identify the following conditions for starting a trace from a tile (i,j). Let g(i,j) indicate the 
    // discriminator grid value at position (i,j); then the conditions are:
    //     - g(i,j) != 0; the tile must not be neutral
    //     - j=0 or g(i,j) != g(i,j-1);  the tile directly underneath it must have a different owner and/or connected flag
    //     - the tile must not already be marked as 'processed'
    // 
    // Additionally, there is one more point to be made; the algorithm initially assumes it's tracing CCW around the territory.
    // If it's tracing an inner edge, however, this will actually cause it to trace in the CW direction (because inner edges curve 
    // 'backwards' compared to the outer edges when starting the trace in the same direction). This turns out to actually be 
    // exactly what the renderer needs to render two territory boundaries on the same edge back-to-back (instead of overlapping
    // each other).
    // 
    // In either case, we keep track of the way the outline curves while we're tracing to determine whether we're going CW or CCW.
    // If at some point we ever need to revert the winding order or external code needs to know about it explicitly, then we can
    // do this by looking at a curvature value which we define to start at 0, and which is incremented by 1 for every CCW turn and
    // decremented by 1 for every CW turn. Hence, a negative multiple of 4 means a CW winding order, and a positive one means CCW.
    
    const int TERRITORY_DISCR_MASK = (ICmpTerritoryManager::TERRITORY_CONNECTED_MASK | ICmpTerritoryManager::TERRITORY_PLAYER_MASK);

    // Try to find an assigned tile
    for (u16 j = 0; j < grid.m_H; ++j)
    {
        for (u16 i = 0; i < grid.m_W; ++i)
        {
            // saved tile state; from MSB to LSB:
            // processed bit, connected bit, player ID
            u8 tileState = grid.get(i, j);
            u8 tileDiscr = (tileState & TERRITORY_DISCR_MASK);

            // ignore neutral tiles (note that tiles without an owner should never have the connected bit set)
            if (!tileDiscr)
                continue;

            bool tileProcessed = ((tileState & ICmpTerritoryManager::TERRITORY_PROCESSED_MASK) != 0);
            bool tileEligible = (j == 0 || tileDiscr != (grid.get(i, j-1) & TERRITORY_DISCR_MASK));

            if (tileProcessed || !tileEligible)
                continue;

            // Found the first tile (which must be the lowest j value of any non-zero tile);
            // start at the bottom edge of it and chase anticlockwise around the border until
            // we reach the starting point again

            int curvature = 0; // +1 for every CCW 90 degree turn, -1 for every CW 90 degree turn; must be multiple of 4 at the end

            boundaries.push_back(STerritoryBoundary());
            boundaries.back().owner = (tileState & ICmpTerritoryManager::TERRITORY_PLAYER_MASK);
            boundaries.back().connected = (tileState & ICmpTerritoryManager::TERRITORY_CONNECTED_MASK) != 0;
            std::vector<CVector2D>& points = boundaries.back().points;

            u8 dir = TILE_BOTTOM;

            u8 cdir = dir;
            u16 ci = i, cj = j;

            u16 maxi = (u16)(grid.m_W-1);
            u16 maxj = (u16)(grid.m_H-1);

            while (true)
            {
                points.push_back((CVector2D(ci, cj) + edgeOffsets[cdir]) * TERRAIN_TILE_SIZE);

                // Given that we're on an edge on a continuous boundary and aiming anticlockwise,
                // we can either carry on straight or turn left or turn right, so examine each
                // of the three possible cases (depending on initial direction):
                switch (cdir)
                {
                case TILE_BOTTOM:

                    // mark tile as processed so we don't start a new run from it after this one is complete
                    ENSURE(!(grid.get(ci, cj) & ICmpTerritoryManager::TERRITORY_PROCESSED_MASK));
                    grid.set(ci, cj, grid.get(ci, cj) | ICmpTerritoryManager::TERRITORY_PROCESSED_MASK);

                    if (ci < maxi && cj > 0 && (grid.get(ci+1, cj-1) & TERRITORY_DISCR_MASK) == tileDiscr)
                    {
                        ++ci;
                        --cj;
                        cdir = TILE_LEFT;
                        curvature += CURVE_CW;
                    }
                    else if (ci < maxi && (grid.get(ci+1, cj) & TERRITORY_DISCR_MASK) == tileDiscr)
                        ++ci;
                    else
                    {
                        cdir = TILE_RIGHT;
                        curvature += CURVE_CCW;
                    }
                    break;

                case TILE_RIGHT:
                    if (ci < maxi && cj < maxj && (grid.get(ci+1, cj+1) & TERRITORY_DISCR_MASK) == tileDiscr)
                    {
                        ++ci;
                        ++cj;
                        cdir = TILE_BOTTOM;
                        curvature += CURVE_CW;
                    }
                    else if (cj < maxj && (grid.get(ci, cj+1) & TERRITORY_DISCR_MASK) == tileDiscr)
                        ++cj;
                    else
                    {
                        cdir = TILE_TOP;
                        curvature += CURVE_CCW;
                    }
                    break;

                case TILE_TOP:
                    if (ci > 0 && cj < maxj && (grid.get(ci-1, cj+1) & TERRITORY_DISCR_MASK) == tileDiscr)
                    {
                        --ci;
                        ++cj;
                        cdir = TILE_RIGHT;
                        curvature += CURVE_CW;
                    }
                    else if (ci > 0 && (grid.get(ci-1, cj) & TERRITORY_DISCR_MASK) == tileDiscr)
                        --ci;
                    else
                    {
                        cdir = TILE_LEFT;
                        curvature += CURVE_CCW;
                    }
                    break;

                case TILE_LEFT:
                    if (ci > 0 && cj > 0 && (grid.get(ci-1, cj-1) & TERRITORY_DISCR_MASK) == tileDiscr)
                    {
                        --ci;
                        --cj;
                        cdir = TILE_TOP;
                        curvature += CURVE_CW;
                    }
                    else if (cj > 0 && (grid.get(ci, cj-1) & TERRITORY_DISCR_MASK) == tileDiscr)
                        --cj;
                    else
                    {
                        cdir = TILE_BOTTOM;
                        curvature += CURVE_CCW;
                    }
                    break;
                }

                // Stop when we've reached the starting point again
                if (ci == i && cj == j && cdir == dir)
                    break;
            }

            ENSURE(curvature != 0 && abs(curvature) % 4 == 0);
        }
    }

    return boundaries;
}