Mercurial > hg > blitz_condensed
annotate src/EDU/oswego/cs/dl/util/concurrent/Semaphore.java @ 28:4b2f23b6feca
Add a schema compatibility check to indexer that parallels storage.
author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 12 Jun 2010 10:11:15 +0100 |
parents | 3dc0c5604566 |
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0
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1 /* |
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2 File: Semaphore.java |
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3 |
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4 Originally written by Doug Lea and released into the public domain. |
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5 This may be used for any purposes whatsoever without acknowledgment. |
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6 Thanks for the assistance and support of Sun Microsystems Labs, |
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7 and everyone contributing, testing, and using this code. |
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8 |
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9 History: |
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10 Date Who What |
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11 11Jun1998 dl Create public version |
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12 5Aug1998 dl replaced int counters with longs |
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13 24Aug1999 dl release(n): screen arguments |
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14 */ |
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15 |
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16 package EDU.oswego.cs.dl.util.concurrent; |
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17 |
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18 /** |
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19 * Base class for counting semaphores. |
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20 * Conceptually, a semaphore maintains a set of permits. |
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21 * Each acquire() blocks if necessary |
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22 * until a permit is available, and then takes it. |
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23 * Each release adds a permit. However, no actual permit objects |
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24 * are used; the Semaphore just keeps a count of the number |
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25 * available and acts accordingly. |
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26 * <p> |
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27 * A semaphore initialized to 1 can serve as a mutual exclusion |
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28 * lock. |
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29 * <p> |
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30 * Different implementation subclasses may provide different |
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31 * ordering guarantees (or lack thereof) surrounding which |
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32 * threads will be resumed upon a signal. |
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33 * <p> |
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34 * The default implementation makes NO |
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35 * guarantees about the order in which threads will |
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36 * acquire permits. It is often faster than other implementations. |
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37 * <p> |
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38 * <b>Sample usage.</b> Here is a class that uses a semaphore to |
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39 * help manage access to a pool of items. |
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40 * <pre> |
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41 * class Pool { |
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42 * static final MAX_AVAILABLE = 100; |
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43 * private final Semaphore available = new Semaphore(MAX_AVAILABLE); |
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44 * |
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45 * public Object getItem() throws InterruptedException { // no synch |
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46 * available.acquire(); |
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47 * return getNextAvailableItem(); |
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48 * } |
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49 * |
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50 * public void putItem(Object x) { // no synch |
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51 * if (markAsUnused(x)) |
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52 * available.release(); |
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53 * } |
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54 * |
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55 * // Not a particularly efficient data structure; just for demo |
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56 * |
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57 * protected Object[] items = ... whatever kinds of items being managed |
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58 * protected boolean[] used = new boolean[MAX_AVAILABLE]; |
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59 * |
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60 * protected synchronized Object getNextAvailableItem() { |
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61 * for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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62 * if (!used[i]) { |
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63 * used[i] = true; |
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64 * return items[i]; |
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65 * } |
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66 * } |
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67 * return null; // not reached |
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68 * } |
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69 * |
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70 * protected synchronized boolean markAsUnused(Object item) { |
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71 * for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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72 * if (item == items[i]) { |
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73 * if (used[i]) { |
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74 * used[i] = false; |
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75 * return true; |
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76 * } |
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77 * else |
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78 * return false; |
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79 * } |
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80 * } |
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81 * return false; |
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82 * } |
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83 * |
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84 * } |
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85 *</pre> |
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86 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>] |
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87 **/ |
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88 |
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89 |
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90 public class Semaphore implements Sync { |
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91 /** current number of available permits **/ |
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92 protected long permits_; |
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93 |
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94 /** |
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95 * Create a Semaphore with the given initial number of permits. |
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96 * Using a seed of one makes the semaphore act as a mutual exclusion lock. |
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97 * Negative seeds are also allowed, in which case no acquires will proceed |
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98 * until the number of releases has pushed the number of permits past 0. |
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99 **/ |
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100 public Semaphore(long initialPermits) { permits_ = initialPermits; } |
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101 |
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102 |
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103 /** Wait until a permit is available, and take one **/ |
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104 public void acquire() throws InterruptedException { |
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105 if (Thread.interrupted()) throw new InterruptedException(); |
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106 synchronized(this) { |
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107 try { |
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108 while (permits_ <= 0) wait(); |
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109 --permits_; |
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110 } |
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111 catch (InterruptedException ex) { |
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112 notify(); |
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113 throw ex; |
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114 } |
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115 } |
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116 } |
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117 |
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118 /** Wait at most msecs millisconds for a permit. **/ |
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119 public boolean attempt(long msecs) throws InterruptedException { |
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120 if (Thread.interrupted()) throw new InterruptedException(); |
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121 |
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122 synchronized(this) { |
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123 if (permits_ > 0) { |
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124 --permits_; |
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125 return true; |
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126 } |
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127 else if (msecs <= 0) |
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128 return false; |
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129 else { |
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130 try { |
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131 long startTime = System.currentTimeMillis(); |
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132 long waitTime = msecs; |
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133 |
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134 for (;;) { |
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135 wait(waitTime); |
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136 if (permits_ > 0) { |
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137 --permits_; |
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138 return true; |
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139 } |
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140 else { |
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141 waitTime = msecs - (System.currentTimeMillis() - startTime); |
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142 if (waitTime <= 0) |
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143 return false; |
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144 } |
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145 } |
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146 } |
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147 catch(InterruptedException ex) { |
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148 notify(); |
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149 throw ex; |
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150 } |
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151 } |
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152 } |
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153 } |
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154 |
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155 /** Release a permit **/ |
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156 public synchronized void release() { |
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157 ++permits_; |
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158 notify(); |
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159 } |
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160 |
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161 |
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162 /** |
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163 * Release N permits. <code>release(n)</code> is |
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164 * equivalent in effect to: |
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165 * <pre> |
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166 * for (int i = 0; i < n; ++i) release(); |
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167 * </pre> |
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168 * <p> |
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169 * But may be more efficient in some semaphore implementations. |
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170 * @exception IllegalArgumentException if n is negative. |
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171 **/ |
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172 public synchronized void release(long n) { |
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173 if (n < 0) throw new IllegalArgumentException("Negative argument"); |
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174 |
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175 permits_ += n; |
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176 for (long i = 0; i < n; ++i) notify(); |
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177 } |
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178 |
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179 /** |
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180 * Return the current number of available permits. |
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181 * Returns an accurate, but possibly unstable value, |
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182 * that may change immediately after returning. |
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183 **/ |
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184 public synchronized long permits() { |
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185 return permits_; |
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186 } |
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187 |
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188 } |
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189 |